Hey i'm kinda new too, i played lots of hours, but i never played the first BG and i'm still way behind the other guys, but since i didn't seem to have had any problems with battles so far (playing normal difficulty) i thought perhaps i could give you a few hints
First off, you got someone to heal in the party i take it? (The druid girl you free) - I don't know if anything happens if you spend too many days sleeping down there, but sleep to replenish your spells.
Second sometimes it's a good idea not to just run through the dungeon but send a spy head, someone who can sneak around the corner and report back to the group. You could send your thief
When you spottet some enemies ahead, you could switch to missile attack weapons, sneak close before they see you, send the melee guys up in a run while the missile guys deal out arrows etc. Also i found a spell of cloud kill, fireball etc. usefull when you find a room filled with nasty little goblins and the like
Don't be afraid to use your spells, fire away! When you run out of spells... just rest and you're ready to go again