You're forgetting that players aren't swinging random sticks, but magical weapons with many powerful enchantments. You could claim that pets are "mundane", but then again, who knows - maybe they were infused with magic from their environment so their attacks are not like your regular dog bite.
It's canon that normal, non-magical warrior was able to actually hurt Sargeras with a magical weapon. The biggest, nastiest superboss so far could still be damaged by physical attacks - and yet random elemental would be immune? How that's for "realism" and "rpg elements"?Technically there's one more option:
C class is affected and much more powerful than it would normally be... so they are stacked while everything else is benched.