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If someone throwing fireballs (or otherwise messing with real physics) is enough to stop your suspension of disbelief, it's probably just badly-written fantasy. In fiction, the author (often implicitly) decides the rules of the world, up to and including the rules about physics. A competent author writes in such a way that (most) readers accept their rules.
If the problem exists on your end, rather than the author's, I'd advice you to either tell yourself that your laws of physics are not their laws of physics or to try to enjoy the work on a more emotional level. That is, try to relate more to the emotions of the characters, rather than the action. Being said because your friend was killed by a stray fireball is the same kind of sad as your friend being killed by a bullet.
I can enjoy fantasy sure. But it was easier when I was younger. Nowadays I can't stand magic that has insufficient structure ("you the reader haven't studied magic so I have to explain it in laymans terms" doesn't cut it for me). I liked Rothfuss' Name of the Wind as there is logic and depth in it and clearly understood limitations - and not just a deus-ex-machina if needed by the author.