Kovel Mansion is the best place to get lots of money and XP with relatively little risk in this game--just bring enough healing spells to get past the wimpy traps and you don't even need a thief. There are a few 1st level thieves who pop in and try to backstab you, but at AC-3 Overman doesn't get hit all that often. Even less often after looting the items cache in the northwest corner, where I ignore the random items (Scimitar +1? Not with 16 slots on one guy) and go for the Shield +2, bringing my AC to -4 (Banded Mail +1, Shield +2, 18 DEX).
I adore the handlebar mustache. I'm sure they thought nothing of it at the time but the thief looks so 80s now.
The only somewhat tough fight is that with the 3 6th level thieves, and the 15 1st level thieves at the south end of the mansion. as you can see, you get surrounded by thieves, and given that they can backstab you for double damage (or triple in the case of the 6th level thieves) you can get TPK'ed. The smart thing to do is to Hold Person the 6th levels (even if you don't get all of them you will usually get 1 or 2) and Sleep any aggregate of 1st levels. Once their numbers are down somewhat, and the dying ones have died (this takes a few rounds), you can help things along with Animate Dead.
Most of you will be too young to remember the D&D Satanic Panic of the 1980s, but in addition to heavy metal music, people were afraid of D&D turning kids to satanism. TSR responded by removing the 'demons' and 'devils' from its games (they were relabeled 'tanar'ri' and 'baatezu') as well as morally dubious characters like assassins. It is thus likely that, in response to this edict, SSI got rid of Animate Dead in Curse.
The spell reanimates the cleric's level of zombies. It only works on humans, not humanoids (unlike Hold Person in this game), but in cases with many low-level human enemies like this one it can help swing the battle. See that 1st Lvl Thief with his name in blue (indicating an ally?). He's not charmed (or resting), he's dead (and animated). Your zombies will add macabre reinforcements to your party, which is obviously useful in a solo situation.
After holding, sleeping, and animating, Overman has just barely survived the fight. However, you have to kill 21 thieves to rest, and between the 15 (the 6th level guys don't count) here, the 5 at the items, and a few in random encounters, that number has been crossed, and with the thieves cleared, he can now rest inside the mansion.