Do you have the spare skill points to put a couple points into Hammer, so one character can cast that repeatedly to stun him, and one concentrate on doing damage?
Assuming you have the act 1 summoning doll and a crossbow, did you have it use power arrows (if possible), rather than normal arrows? The Convert Arrow skill isn't available for power arrows until act 3, but I don't recall if they start showing up earlier as random loot or for purchase.
I didn't have too much trouble defeating the shaman on level 18. In the cellar you need to get him down to about 1/4 health before he teleports you to another location.
What type of weapons and spells are you using? At level 18 (at least in my game) the shaman is most resistant to poison, shadow and bone damage, and fairly resistant to elemental damage. He is pretty much neutral to conventional damage, and slightly weak against ethereal damage.
Do you have any stat boost potions, that might give you a boost in damage, or otherwise a bit of an edge?