[Amergin]

i dont think ive ever had a character that wasnt at least part thief.

first, even with the best weapons in the game you're really going to suck in combat because of the armor restriction, hp's barely better than a mage, and a THAC0 worse than priests'. iow, thieves are wimps beyond the backstab.

i would get your find traps up to 90; going solo you dont have any other option and your hp's and saves arent good enough to try and absorb the effects.

then improve what you already do well; get your set traps as high as you want. 90 will do but 100 will guarantee no traps fail.

after those two are max'd, work on stealth. fyi, hide in shadows controls how likely you are to hide, and move silently controls how well you can stay hidden while moving, so if you're patient and can afford to attempt hiding a few times, concentrate on move silently first. also, the maevar quest will get you some items that boost stealth so aim for the 70-80 range. once you get them youll be fine.

stat wise, an elf or halfling are always the best thieves as they can have a 19 dex. while both 18 and 19 give -4 to AC, the 19 gives an extra +5% to thief abilities as well as +3 to missile weapons instead of the +2 for a 17/18 dex, and short bow should be your main weapon. dont bother with strength (12 is good enough); there are tons of items and potions throughout the game, and spending stat points to get an 18 (+1 thaco, +2 damage) is wasted on a theif. definitely a 16 con and a high wis to help against mind spells. there's an item early to take care of cha so dont bother.

basic tactics: set traps, hide, backstab the toughest bad guy, run back to the traps while shooting with your bow. hide/backstab/retreat+shoot/repeat.