I don't have enough mana to cast hammer even once!
Not even at low power? Hmnnnnnn.
Obvious approaches:
1. Buy potions of intelligence and take same - I'm not sure how long they last but but you can stock up with a few. Should increase intelligence by enough to give plenty of mana.
2. Check the stores for items with +int or + mana - maybe crap you sold to get better stuff. Buy them for this fight.
3. Alternative to buying potions - give yourself some points in extract and use alchemy to make potions.
4. If you have any charm slots available (preferably on gear you don't plan on keeping all that long) try sticking a mana (Icen) or int (Koor) charm in. The green charms don't do much but they are cheap and may do enough.
5. If you can use the battlefield to go up a level this will let you raise intelligence a few points. Or clear out a dungeon you haven't bothered with (if any) for the loot such as potions of int, gear raising int and mana etc.
6. If all else fails play unfair. Every time you get a good whack in and the shaman goes down nicely in hit points, save. With enough restores you will hit every round ,smile> and he will die real fast for you too.