I assume you mean something that will change, say, an incoming fire attack into a cold attack?
Hmmmm.
Sounds like a Transform with a Damage Shield. My first impulse was that a Cosmetic Transform was too "small". However, considering the new extremely high cost of Damage Shield, that sounds about right.
Then the dice of Transform could be matched against the BODY of the incoming attack. If the Transform wins (or ties), the attack is changed.
If you want it to be partially effective against powerful attacks, slap a small Advantage to that effect on the construct (+1/2 sounds about right).
Remember, you can always define Advantages and Limitations however you need them. HERO never tells you "you can't do this" (with the exception of some absolutes) - it merely gives a point cost.