High teens to low twenties should be good enough to do the cursed abbey (perhaps a little more for a mage, depending on what spells you have). You may want to clear the surrounding area first, if you haven't already, since zombies are fairly slow, and relatively easy to take out from a distance (magic or bow). Alternately, you could go exploring somewhere else.
You may want to keep an eye out for Spell Shield and Aura of Guarding spell books, to absorb magical and physical damage, respectively. Spell Shield is especially handy against poison damage (ie fighting zombies) since the damage comes off the shield and your character doesn't jerk around unable to defend or attack effectively.
There are a limited number of opponents in the game, but you can resurrect a group of opponents, or one strong one, then run or teleport away so they turn hostile, and return to kill them again. This can get boring fast, so if you try it at least choose targets which are fairly strong, always drop loot, or which have a chance of dropping charms (ie the lizards in the sewers or Dark Forest).
Killing monsters gives more experience when you are at a relatively lower level, while quest rewards give greater experience at higher character levels. If you clear an area of monsters before finishing up quests, you should get slightly more experience than doing the reverse (don't put off doing quests if you get tired of hack and slash, though).
The Wisdom skill will help you level faster, and can give you an extra level by the end of the game.
Wisdom - is it worth the skill points?There are a couple level bonuses in the game. If you wait until you just go up a level to trigger them you will get the best effect, and the longer you wait (higher character level) the greater an experience boost it will be.
Most people finish the game with a character level in the mid forties to low fifties.
Kaan's Mage GuidePlaying a mage character...Free skills, and spell books you can buy
