Fireball projectiles bug?

It's about normal enemies on normal maps (T12-14), either skeletons or goatmen, who launch fire projectiles and kill my character in 4 hits. I have >65% both attack and spell block and lucky chance to block from the Gladiator class. I have 4k life and 75% fire resistance, but somehow EVERY projectile hits me and takes away at least a quarter of my life. I charged 6 skeleton mages with molten shell on, in the sewers, where there's no space to dodge left and right, and they still killed me in a synchronous one-shot. Is this game deliberately trying to make no sense or is it a bug and you can as quickly as possible fix it? I have no problems with any other projectile types, lightning or cold, I block them almost 100% of the time and they don't deal such damage, not even close. Can I continue this game and try to gather experience (tediously, but gather it nonetheless) so I can reach further levels? Or do I have to gain 5% xp per map and then have it all wash away from some unexplainable surprise, all the time? Please let me know, ok?

I forgot to add that yesterday I stepped on the burning ground generated by bismuth ore and I was instantly dead as well. There must be some sort of bug, since I have decent life and max resists. Such things shouldn't kill me instantly.

I think I've pinpointed the problem. It relates to fire DoT. And possibly with the block chance, sometimes. I went again through the area with the skeleton mages and I could hear the projectiles being blocked, a lot of times, so maybe sometimes I'm just unlucky in a perfect row? But the issue is with the fire DoT, because when the massive damage was done I saw that I was ignited. So, there's a connection with the burning ground from the bismuth ore. But how can some puny mages, on a map with no advantage, be so deadly, when I can stand next to most of the bosses and rare enemies and rarely receive a hit or, when I do, it doesn't do such damage? Someone please educate me why, so I can try to avoid it somehow, or investigate this issue, so it can all make sense again and people like me can at least minimally enjoy this game.