Worm Hunter Achievement?

[Chezus]

Hey guys! I'm trying to get the "Worm Hunter" achievement. When I see one of those giant worms, I alway's spam my magic explosion spell. But then, the worm has no health, but he is still alive... :/
Eventually someone can help me and/or give me tips. :)

[Lions_Den]

I think you have to hit him with a fireball for the killing blow. Spam the explosion to get him down and then hit him with a fireball.

[Chezus]

Thank you two, for the fast and nice answer! I'll try that when I see the next worm! :)