Personally I found the cursed abby tougher than the orc camps. I cleared the orcs at level 18 with a warrior and did the cursed abbey at level 20 and found that tougher. I think the main reason was you can't freeze skeletals but orcs can be frozen. Also the archers that use fire arrows raise havoc with low fire resist, they also stun you a bit when they hit.
For the Orcs, one point in freeze works well, just be sure to take out any orc drummers 1st. Life leech can also drop the HP's of an Elite giant orc fast, plus the life you drain gets added to your HP's. The toughest orcs are in the most SW corner. Level 13 is too low for the elite orcs or inside the abbey.
Like Raze said the zombies are slower. Have you done that quest for Marcus in market square yet? After you finish that quest is a good time to do the sewers below Rivertown. I could handle those at level 13 and it gave good xp.
