Try it? He will.

The energy blast doesn't punch a hole through the back of one of Agni's heads, he's not weak to that particular element (and good thing for him SecUnit doesn't trend towards ice), but that doesn't mean it doesn't hurt, the smell of burning a strange thing to associate with a demon of fire. How much damage is done is obscured by Heat immediately snapping with intent to bite the offending arm and arm-set gun - clean off if he can get a good grip and put enough force behind it. Harder said than done when this form has no neck to speak of. And then heave in an effort to throw SecUnit off so he can better put his claws to use. Heat only really has two things going for him, raw power and durability. At least it means he can't suddenly burst into flame .. not here, not anymore.

There is exactly one authority Heat will obey these days, and it's not SecUnit. Its opinions and wishes simply didn't process as being even remotely important, just another enemy that didn't fall in line when its leader was killed or an upstart rookie that didn't know its place.

That there's a third secret option won't be believed for a while.