I have found that the very best tactic to kill the Doom Guard (and Battle Horrors as well) is to immediately cast entangle on them. It would be most effective if you have two party members who can cast it (for example if your own PC and Jaheiara cast entangle). If he saves vs. spells, try again quickly, and if he's already too close, reload and try again. Now, as soon as you have him entangled, he can't reach you with that big nasty fiery sword of his
So what you do is equip all your characters with missile weapons and shoot away. Arrows of ice worked well, if I remember correctly. Also have every mage cast as many magic missiles as possible, these will affact the Doom Guard as well (also use Wands of MM).
Hope this helps. With this tactic I have found that you can kill him even if you aren't very advanced in level.
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Melusine, Archbabe of the Order of the Holy Flame and the Laughing Hyenas, &
Official Entertainer Elf of the BG2 Bar
Your voice is ambrosia