Gremor: great for trashfight raids, great in HIDDEN epic arena team, no other uses. WOULD be good in arena if not for the 5 frontliner meta that is filled with OP divers!!! Great in some very specific stages in skyfortress, but gremor is still squishy. Great for Cof but kong clones are his weakness as are tanyas. morfirs etc... he takes to much damage to bring from start to finish
Vulko: great vs raidboss ch 16, 17, great in any epic arena team at 7 stars, great in arena, great in skyfrotress on basicly any stage when 7 stars. Great when paired with MIKA who is the best pet in the game. One of the best heroes for CoF, can use him start to finish
I personally would choose Vulko 100 out of a 100 times, because at 7 stars he is an arena god, a CoF god and at full academy and runes and a mika he will make everyone cry... A 7 star Vulko is nearly unstoppable.