I think there are two options to do this:
a) Trigger the spell yourself -> this gives you the most "freedom", but requires a lot of work as you need to completely trigger the spell yourself. Then you can damage enemy using attack type spells damage type universal (or something like that), or make other checks to either deal physical or magical damage, etc.
b) I think this will also work as a "physical" spell if you make the Firebolt a hero ability that requires level 2 or more (didn't test this ever myself, but I think there was a topic here where people discussed similar thing and making it an ultimate ability solved it)