Naga has a good sized path he roams I tried to point it out above. Naga Like all moria trolls gets this great buff that adds +%10 mele damage -%10 incoming mele damage (so you hit him for less) -%10 attack duration (so he attacks you faster) the buff keeps stacking the longer the fight goes on. So your best bet is to kill him as fast as possible. If you pull the adds around him carefully you can fight Naga one on one, or if you wait for him to walk up the stairs or down the hallway.