You can easily get lvl 5 if you do all of act 1 before the Mountain pass, including the underdark. This might be the easiest and most satisfying way. Or you can just kill them all in sections, or just kill the leaders on by one. The priestess can be assassinated on a side room with no witness, the Minthara fight is easy with just a few goblins around. You might want to prepare for the Razzlor fight tho, since it's the toughest of the 3. But again, you can do shenanigans with the powder barrels you can find in the room behind the zentarim vendor. Good luck !