If you resurrect a group of opponents (or one strong one) then run or teleport away, they will turn hostile and you can return to kill them again for more experience and/or loot. This can get boring fast, though, unless you just resurrect and leave them for the next time you happen by (say with groups of lizard men in the Dark Forest, who have a small chance of dropping gold charms). I played with this a bit in Iona's dungeon, with a guardian by the teleporter pad (that always dropped something half decent when killed). Mostly I just used this technique to make sure I went up a level before getting a level bonus (there are a couple in the game), to maximize the experience points received. I maxed Resurrect with purchased spell books.

The Wisdom skill will also give you a bit of extra experience (though not enough to get over level 53 by normal means): Wisdom - is it worth the skill points?
As described in that link, the experience required for each level goes up by the cube of the level, so even killing NPCs you would not be likely to get to level 54 or 55.

Completing quests gives more experience at higher character levels, so within reason you may wish to explore and clear opponents first, before finishing quests. You don't want to forget about main plot quests, though, or end up with so many open quests you don't know where to start.

Opponents give more experience if they are at a higher level, and less if they are lower. This can be used to level faster, but when you do get back to taking out the weaker opponents, you may get little or no experience.
Without leaving Aleroth, you can reach level 4 before you enter the catacombs.
Explore / loot around Aleroth and kill any rats, chickens or cows that you want (50 exp on level 1, plus meat which heals a small amount), but you may want to delay completing any quests. At level 1, if you defeat the zombie Jake (the graveyard would be easiest), you will get enough experience points to bring you up to level 3. This requires a lot of hit and run with a bow, or a moderate amount using Meteorstrike. Defeat him a second time (in his cellar) and you get to level 4.

If you want to kill NPCs, you can do so (if they are unprotected) easily by casting an area effect spell that doesn't need a target (ie Poison Cloud) near them but not directly on them. That way you do damage, but they don't turn hostile and you don't loose any reputation.
When I tried killing Otho's pigs (after saving) he attacked quite quickly and effectively (no lightning resistance to speak of). In any case, do not kill Joram (if that is possible), since you need either Joram or Otho for a main plot quest later. The latest patch, or an already-updated re-release version, should have most or all of the essential NPCs protected, though getting your reputation low enough that no one will talk to you would probably cause problems, as well.