1. Even at skill level 2 (and you can get 5 from the hermit in act 2) and with low intelligence the hammer spell (shaman magic, fittingly enough) may turn the fight around for you. Have one character cast it repeatedly while the other pounds away - the hammer usually paralyzes the enemy for 2-3 rounds which is more important than the damage it does.
2. Are your characters using nice sharpened weapons - that makes a big difference. Also enemies like the shaman are why those big 2 handed weapons are in the game - you don't need a shield here you need to do major damage and you need to do it fast. The magic sword you should have found right at the start of act 2 is good even if you don't have any better weapons yet. You might switch to 2H (if you aren't 2H already) and sharpen it as high as you can (beware - if you are patched to 1.45 sharpen requires much higher agility than 1.44 - see separate thread).
3. This is a fight warriors will do better at - with two warriors with top quality weapons I killed the shaman in two hits once - the SOB was yakking about how he was too powerful for me but LOL he was already dead. With two casters I had more problems but prevailed alternating the hammer spell and hitting him with mediocre (but very sharp) weapons.
