About mages and spiders. This is supposed to be a thief so prowling and backstab should be fine. When there are walls, sneak to the corner, enter combat with turns, use your turn to get around the corner and kill it.
So your party will be more annoying than helpful. A thief best works alone.

Traps are also in the tunnel to the bates mension, both dungeons for the dragon blood quest, Shuylers and son dungeon and some more.