I think we're moving off point a little.
Based on the SFX, we have an attack that manifests as a gaseous substance. This gaseous substance only works against a person that has a sense of smell. If a person can fight off its effect, they are immune to its effect for a day.
An attack that works based on their ability to smell does not need for the subject to breathe normally, so having Life Support: Does Not Breathe is not protection from the attack unless the SFX of the protection affects how they smell; an example being a space suit or hazmat suit. While the nature of the monster he's converting isn't clear based on the OP, being in a vacuum does not protect you from the scent, but at the same time, most people don't try smelling in a vacuum either. Holding one's breath does not protect from noting a scent, just makes it a lot harder to smell since humans generally need to breathe to smell, although you could hold your nose to prevent smelling the substance.
So, we have an Entangle with an Area of Effect of 4 meters. The inability to smell is rather rare so I don't feel IMO that warrants a Limitation for the power but if we want to represent the Save Throws mechanic in this power, that does warrant a Limitation.
In a game like Starfinder, Pathfinder, and D&D, the character's ability to resist effects is based mainly on their experience level, but since the Hero System is a point based system we need to handle things differently. The easiest way to represent this would be to use the Requires a [Skill] Roll mechanic but this is inverted in that it a roll to see if the power fails as opposed to succeeding and the difficulty to resist the power is based on the Campaign needs. Being based on Constitution seems appropriate, but the target number is the issue now.
Resists with a Constitution Roll
We can start with the CON roll being worth a -1 limitation. I feel this is mechanically the same as using your 11- roll from Requires a Roll and the additional 1/2 limitation for the -1 to roll per 5 AP from the table on CC 114 or 6E1 389. If you want it to be easier or harder to resist, we can take use the table as a guide.
Penalty to CON Roll Limitation
-3 3/4 less limitation
-2 1/2 less limitation
-1 1/4 less limitation
Bonus to CON Roll Limitation
+1 1/4 more limitation
+2 1/2 more limitation
+3 3/4 more limitation
+4(or more) 1 more limitation
I'd love to hear your comments on this solution