If that's the rat battle behind the rat hole, that IS a tough battle relative to your character's level... use every spell you've got.Rats and bats seem to dodge attacks very well, and there's a lot of them in the early parts of Wiz6. I agree with DrSlop that a Bard would serve you very well by putting those rats & bats to sleep. (If you haven't gotten far, consider rolling a Bard to replace your Samurai, and have the Bard change to Samurai at higher level for the added melee power.) But with the party you have your best bet is for your Samurai to learn the sleep spell as soon as possible and optimize your attacks and weapons to provide the best + to hit possible. (Or find/buy Faerie Dust... I don't remember if QueeQueg sells it.)
Look up all the attack modes (swing, thrust, bash, etc) in the manual and use the ones that give you the best + to hit modifier (usually swing). Also look for weapons from QueeQueg that have a bonus to hit. Rapiers have +1 to hit and the Sword of Striking you find in one of the first chests has +2 to hit. The Bullwhip from the Queen's room is +1 to hit also, plus the Lash attack I believe has an inherent +1 to hit. The Skull Dagger from the Pirate's Den (after you out-drink or kill the pirate) is +1 to hit. Your Samurai's Katana and Monk's Bo have +1 to hit also. Avoid the Bastard Sword (good damage, but -1 to hit). Hopefully by putting all those together you can come up with a party weapon configuration where as many members as possible have + to hit weapons and you can actually hit those rats & bats occasionally!