The Mage's sleep spell and fireball spells are the shit. I also go with the one-in-front combat fuck style. The fighter's got a one-handed weapon and shield, in the first round the priest casts a lower AC spell on him. Most attacks are gonna bounce off the fighter with that combo. The rogue plinks away (I haven't found backstab useful, either) and the mage uses sleep or fireball as appropriate. Sleep is awesome because guys who are sleeping are easy to hit and take double damage. Much like Myke's sleep spell effect in our last 4th ed game, in fact....
The basement is kind of a red herring right now. You're better off exploring the 2nd floor and doing the heirloom quest first. One tip, you'll want one person to have the gambling skill you can get at 3rd level, see if any of your guys can get it. If not, get the pre-made mage douchebag and level him up to 3 and use him for a little bit. I'm not sure but I think all the 500 xp skills are useful while the 1000 xp skills are just jokes.
Also, did you find the statue with the bare head yet? I wasted a lot of time looking for like a magical hat or some shit to use with it before I found out that you can just slap any old helmet on it. That opens up a really big chunk of the dungeon.