LoneWolf

Like anything else in the game, it depends on the special effect, and what you want the pain to do.
 
If the goal is to knock someone out some form of Stun only attack will work.  This could be an NND or other exotic attack or just a plain Stun only attack.
 
If you want the pain to cause other effects, you need to build those using other powers.  One way would be to use a limited form of mind control.  A single command drop the item you are holding because it is causing you great pain would simulate this.   Another way would be a change environment requiring an ego roll to continue your action.  Like I said it depends on what effects the pain causes.   
 
This question is kind of like asking, how would you build an electrical attack.   There are many right answers to both of these questions. 

Most super heroes have a CON of at least 18.  With a CON that high you probably rarely get sick anyways. That same CON will mean your character probably handles the illness better than normal.  

You might want to look over this thread Invisibility to Detect

I have to agree with Hugh on this.  What he is saying is a fundamental concept of the Hero System.  A limitation that does not actually limit the power should not reduce the cost of the power.   This has been explicitly stated in every edition in one form or another.  
 
The one thing that should be brought up, is that you don’t have to actually take body for it to have an impact on the game.   Body also is also used to determine knockback.  If a character has stun only, they can still be knocked around by the attack without actually taking BODY.  If the campaign is using knockback than Stun only should be worth some limitation.  At that point the argument is not whether it is a limitation, but rather how much it is worth. 

Don’t get me wrong I am a big fan of damage negation, but it is better at creating a high threshold of damage.  It is great for characters that are immune to damage below a specific point.  Damage Reduction is better for the character that is not immune but can take a lot of damage and still keep functioning. It really depends on how you want your character to work.  If you want to be immune completely immune to small arms but take damage vs larger attacks DN works better.  
 
A character with moderate defenses and 50% DR can probably take one hit from DR Destroyer and still be able to stay in the combat.  That same character can be nicked and dimed by enough agents. The character with the same defense and -6 DC of DN will completely ignore the agents, but likely be taken out by the same single hit by DR Destroyer.  Both concepts are equally valid.        
 
If you scale up the defenses the results show similar results.  With 15 defense vs a 12d6 attack 50% DR results in 13.5 points of damage, -6 DC of damage negation results in 6. At 15d6 the DR damage is 18.75, where the DN is 16.5.  At 18d6, the DR damage is 24, and the DN becomes 24.  
  
For the same cost the results of DR and DN are fairy similar
 

I have to agree with Hugh on this.  What he is saying is a fundamental concept of the Hero System.  A limitation that does not actually limit the power should not reduce the cost of the power.   This has been explicitly stated in every edition in one form or another.  
 
The one thing that should be brought up, is that you don’t have to actually take body for it to have an impact on the game.   Body also is also used to determine knockback.  If a character has stun only, they can still be knocked around by the attack without actually taking BODY.  If the campaign is using knockback than Stun only should be worth some limitation.  At that point the argument is not whether it is a limitation, but rather how much it is worth. 

This falls into the same category as many other limitations - is it limiting in the context of the specific character in the specific campaign? 
 
The pricing question is an issue.  Typically, "resistant" has been a +1/2 advantage.  It doesn't seem like this has been re-evaluated with the evolution of killing attacks.  It was +1/2 when nonresistant defenses did not reduce Stun damage from killing attacks, which made nonresistant defenses considerably less valuable.

If anything, the loss of reducing Drains and STUN damage from KAs suggests that nonresistant is more limiting for DN and DR than for normal defenses (ie moving Resistant Protection to PD and ED). It seems like either nonresistant DN and DR should still reduce STUN from KAs (and perhaps all switches between Resistant and Nonresistant should be repriced at -1/4) or nonresistant DN and DR should be a greater limitation than nonresistant defenses.
 
The -1/2 for "only BOD" or "only STUN" presupposes that reducing each is of equal importance.  Where that supposition is not accurate for the specific campaign or character, I would take issue with the limitation.  In other words, as a GM, if you apply, and I allow, "No BOD" to your Negation or Reduction, then the onus is on me as GM to ensure that the limitation becomes relevant in actual gameplay.  For STUN Only, that means ensuring there are instances where you are at risk of BOD damage not reduced by these defenses. 
 
If I have two players, one who pays full price for Negation (or Reduction) and one who takes a discount for "STUN only", the first player deserves to see some value for the extra points spent.

It is in 6th edition 6E1 page 392.  

I would not allow a limitation for Stun Only defenses unless the character's other defenses leave BOD damage a reasonable possibility.  I don't see that very often.
 
As I see complications and limitations as the player's communication of the kind of challenges they want to face, limiting defenses against BOD means that they want BOD damage to come up as a real threat.

I created a spreadsheet that you can change the base DEF to compare an equal amount of points in DR vs DN.  
DRvDN.xlsx

I created a spreadsheet that you can change the base DEF to compare an equal amount of points in DR vs DN.  
DRvDN.xlsx

I have to disagree with DR 25% being useless.   It is less effective vs low dice attacks, but more effective against high dice attacks.  25% damage reduction and 3 DC’s of damage negation will both mean that a character with it will take a 13d6 attack to stun a character with a 23 CON, given 12 PD.  UP to about 16d6 the results are fairly similar, above 16d6 the damage reduction provides better defense. 
 
If we go up to 50% DR and -6 DC of damage negation the dynamics change.  The Damage Negation provides near absolute protection up to 9d6, but reaches the stun point at 16d6 vs 17d6 for the 50% DR.  Above that point the DR gives much better protection than the DN.
 

I have to disagree with DR 25% being useless.   It is less effective vs low dice attacks, but more effective against high dice attacks.  25% damage reduction and 3 DC’s of damage negation will both mean that a character with it will take a 13d6 attack to stun a character with a 23 CON, given 12 PD.  UP to about 16d6 the results are fairly similar, above 16d6 the damage reduction provides better defense. 
 
If we go up to 50% DR and -6 DC of damage negation the dynamics change.  The Damage Negation provides near absolute protection up to 9d6, but reaches the stun point at 16d6 vs 17d6 for the 50% DR.  Above that point the DR gives much better protection than the DN.
 

Special effect based on real world sciences is usually fairly easy to understand.  Fire or cold is something that is pretty obvious.  Special effect based on rubber science on the other hand is something that needs to be defined before it is allowed in the game.   Cosmic/Primal energy is rubber science so the person creating the vulnerability should be the one defining it. 

Generally speaking, the best way to start building a character in Hero System is to decide what you want the character to be able to do, e.g. its physical and mental Characteristics, Powers, etc., then find what in the system matches those specs best. In your case, you should also consider what you want the character to be like in the powered armor suit, and outside it. Is the suit a part of him, like a modular add-on to his body? Or is it a completely separate, self-contained thing? Does the suit have any independent volition? Is he ever without it? Any difficulties putting it on or off?
 
If you could describe how you want all that to work, it'll be easier for us to suggest appropriate ways to model it.

From what I understand you want an alien robot wearing a power suit.  If that is the case the best way to do that would be to write up the alien robot first but save enough points to build a suit of powered armor.
 
Depending on how the robot is constructed it could have some automation powers but could also be built like a standard character.  The STUN, END and REC can be the special effect of a sophisticated robot with internal power source and repair capabilities. When it takes stun, that represents minor systems being damaged.   You could also purchase some automation powers, but those tend to be powerful, and many GM’s might not allow that. With the remaining points purchase your powered armor.   
 
One thing to keep in mind with the Hero System is that special effect is everything.  Almost any power can represent almost any special effect. There is no power in the Hero System called lighting, but you can use the powers to build it.  Being an alien robot or having a suit of powered armor is simply a special effect.  Think of what abilities the alien has and buy appropriate stats, powers and skill.  Then think what you want the powered armor to do and buy those abilities.    
 

Again, its worth considering schtick.  Letting a character be especially great at one specific thing that sets them apart from the group is fun for the game and usually fits genre: he's so good with a knife he can beat a quick draw gunfighter!  So having a character that's just super, super accurate is okay, as long as they aren't also heavily damaging.  Hawkeye never misses with his bow, okay.  But he also isn't dishing out Thor-level damage, either.

I have to agree with Sketchpad that they are only guidelines.  The title of the chart that you are referring to is actually Character Ability Guidelines table.  I use them to make sure that the characters are within the ranges they should be.  If you scroll down in the book there are a few more tables giving more guidelines.  The characteristic comparison table shows that characters with that range (5-9)  fall within the competent to legendary classifications.  A few pages latter there is a table called Characteristics Maxima Table, which gives the normal human maximums.  That table lists the normal human maximum for ocv/dcv as 8. 
 
As a GM I will use these to set campaign limits.  Those limits are usually a bit higher than the ranges presented to allow some variation and use of tactics.   Even with the limits it also depends on the nature of the character.  A character that relies on skill levels to increase the damage needs more flexibility than one that can reach the campaign DC l limit without needing anything else. If the character has to make a choice between damage and accuracy they are usually less of a problem.  I also look carefully at characters using 2 point skill levels to boost a single attack.  Those are more troublesome than a character that spent more points to achieve the same results.    

I have to agree with Christopher on this.  Hero system has a lot of optional rules that can allow you to run any type of campaign you want.   I have been in many low powered champions games and have never found them lacking.    
 
If you want a gritty granular campaign it is a matter of setting up the campaign parameters and making sure people understand that is the game, you are running.  This usually means imposing campaign limitations on powers especially defenses and stats.   Just because the average SPD in most Champions game is 5 does not mean it has to be that way.   Simply let the players know what is appropriate for the campaign and enforce that.  

It is possible that GURPS handles low end campaigns better than Hero, I don't have enough experience with GURPS to decide one way or another on that.  I just know that Hero works really well for low end in my experience.

I question the premise, I have no problem running low powered games with Hero and have done so for 30+ years.

You might want to look over this thread Invisibility to Detect

Hit locations affect more than just stun.  They also affect the amount of Body the target takes.  The body multiplier is after defenses, so the dynamics are a little different than stun.  A vital hit on a creature that takes no stun does 2x the amount of body after defenses.  
 
The creature with Takes No Stun has a higher chance of taking maximum effect from the hit location than a normal creature.   To the creature with Takes No Stun a head shot and a vital shot have the same effect.   The chance of a head shot is .46%; the chance of a vital shot is 9.72%.  The creature with Takes No Stun has a 10.18% chance of taking the maximum effect. Given the lower defenses of the creature that Takes No Stun it is more likely that Body will get through.  That means a vital shot to a skeleton is a lot more likely to take them completely out of the combat.
 

Most people look at the hit location rules and all they see is the stun multiple.   The Body multiple is what makes it really dangerous.  A vital shot in FH from a decent weapon has a good chance of killing anything.  It is somewhat offset by the locations that halve the Body, but even those can be dangerous. If you are using the disabling rules a hit to the arm or hand can often put your character at a severe disadvantage, even if the Body is cut in half.  Losing the functionality of an arm or hand can be worse than taking damage.   That is one of the reason combat in FH can be a lot deadlier than in Champions.  
 

Already gave them life support. 
 
I agree with Gauntlet on the hit location.  The exact location might be slightly different, but the game mechanics can still work.  A “head shot” might be snapping its spine thereby cutting it in half.