Diablo II, Nightmare/Hell Mode, Undead Dolls

I've been playing Diablo II as I do every 6 months or so and have wanted to know this for a number of years now:

In Kurast; when playing nightmare or hell mode; whenever I kill an undead doll it takes 60-90% of my health. What is this? How can I avoid it? It only happens when I'm using melee attacks to kill them.

The undead doll out of all enemies is practically the only enemy in the entire game that gives me issues, because of this.

The dolls/bone fetishes do an explosion, similar to the necromancers "corpse explosion" skill. It deals a huge amount of physical damage.

If you're a ranged player it should be easy to snipe them from the distance. If you're a melee character, go for -% physical damage gear, like Stormshield, Rockstopper, Vampire Gaze or other to reduce the damage. Also try to aim for high health (obviously).

AFAIK assassins can also block the explosion when they have a high "Weapon Block" skill. Still not something I would try though (at least not in hardcore mode :))

bummzack has the correct answer to your question, but I wanted to add another solution.

I always bring along a merc - I prefer the act 2 nightmare merc with holy freeze aura - and move him around to take out the dolls. The mercs are (or were, last time I played) immune to the dolls' damage, and with proper gear can take them out quickly.

Since mercs can use ethereal weapons/armor without them degrading, it is possible to rather cheaply make some stunningly high damage weapons/armor for them. I tend to favor the runeword with Meditation aura (Insight, Ral + Tir + Tal + Sol), for the high damage and mana regen. Though at higher levels and better gear, mana regen becomes less important.

Be aware that it is lethal to teleport into a pack of dolls, though, because the merc can kill a doll quickly, and with you standing next to him, you will get hurt.

Lastly, of course, I have to impart my favorite gem of wisdom: The best way to not get hurt is to not get hit. Some enemies are best left alone. Dolls are one such enemy.