It’d have to be a pretty huge hailstone to explode your head.
Hail would probably be done better as chip damage-over-time, based on the heaviness of your armor.
Heavy Armor – extremely minor
Medium Armor – somewhat more damaging
Light Armor – the most damaging of all
This way, you don’t require anything special to combat it.
As far as the flesh-eating flies, it would probably come across as a big annoyance like the constant rain is for people now. Just reason to ignore the area completely. Not good practice for a game.
Instead of flies, which seems only appropriate for a swamp or death field type biome, vampire bats popping out at night on occasion could be more devastating, while at the same time easier to combat. Just don’t go out at night in that biome.
Risk going out at night, and the bat frenzy could spawn. Lots of light could steer them away from you as a form of protection. One torch being a mild protection, a bonfire being the best portable protection, with being inside a structure as the best of all.