D&D 5E Spell, Item, or Feat?

[MechaTarrasque]

I would say a boon or very rare item. In 5e, something like this would be a Big Deal, so to keep it that way, it should be hard to get. It is more special if your PC is the only one who can burn a fire elemental.

 

[Gadget]

Late to the party, but there is the spell from Elemental Evil & Xanthanar's called Elemental Bane that removes resistance to a particular damage type and downgrades immunity to resistance. The target also takes extra damage from the chosen damage type as well, iirc.

 

[Perun]

Late to the party, but there is the spell from Elemental Evil & Xanthanar's called Elemental Bane that removes resistance to a particular damage type and downgrades immunity to resistance. The target also takes extra damage from the chosen damage type as well, iirc.

It doesn't do anything for immunity, it just gets rid of resistance to one element on one target (at its base level; it also gives a potentially decent boost to damage (it's per turn)).

 

[Coroc]

I would rule that he has the ability to alter fire into radiant / force or eventually cold. The cold approach is the most logic instead of creating heat he would remove heat so quasi invert his casting.
There are many advantages to that one:
You do not have to construct something which sounds highly illogical.
Since I do not consider this ability worth anything (it is of course but not a feat or such, it just helps your PC not to be useless in some very selected situations), you could do it without concern for balance.
You do not have to alter anything of the rules except the type of element, so introducing it is totally easy and can also easily be shoehorned. It might be some skill your PC has developed by research and therefore can be a good reward for out of session RP.