How come is it when you have two Grand Master fighters beating a monster senseless, the monster turns and tries to bleed-out the thief you then so carefully placed behind it to give the fighters a bit of help with the cripple strike? They must have that built into the AI! Darn--I wish the thief could stay invisible just a bit longer to get that hit in. [I decided on a pure Thief for the challenge--they suck in HoF, but what the hey].

Try a Halfling Fighter/Thief that uses Bows. ;) I think you might like this configuration. The Blacksmith sells an unique Helm that grants an AC bonus of 3, and it is usable only by Halflings. A Halfling F/T will be able to wear it.

They can also wear Full Plate Armour if you so desire, but for practical reasons, I usually stick to Studded Leather Armour because it is a PITA to switch armours everytime I want to lockpick or detect/disarm traps. Besides, they are in the backrow shooting arrows.

If you truly want to stick to a pureclassed Thief, then try this. An Elf Thief that uses a Bow as well.

Thanks for the advice.
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I used an Elf in the past, but I have an Elf Mage and Ranger. Variety is my spice for this gang, so I went Halfling (good magic resistances).
[secret tips coming up]
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I use the Helm on my Gnome Fighter--I cast Speed on 'er and the summoned ones and dive in. My strange goal [using pure characters] for this game is to create an Archdruid (it will take 3 random potions--sacrificing a CON point). I want see just how powerful a +20 WIS Druid can be at LEV 30. It did take forever to roll the big stats for her--but she IS my party leader.
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I'll just give the Halfling the best sling (they get the bonus like an Elf for bows), +1 STR (bone ring), +2 THAC0 (necklace), fast boots, and use 'em mostly to toss rocks and thieve [Although it's fun to get a gang-up-on-you battle going with a sneak attack]. I pump the STR up yet another notch with the Strength spell--it lasts 12 hours, so I take advantage of that. It WOULD be an 18/51 for an Elf--giving that +1 to hit!