Hint #1 - Rahk Zor can be pulled without adds.

#2 - in the tunnels pull mobs quite far back so you dont get conjurer's wandering in mid fight - they have set patrol paths so learn where they are and wait for them - nuke them down as fast as you can - if there are other mobs (most likely non-elites) ignore them till the conjurer is dead.

#3 - after you kill each boss/ open a door - a patrol of 2 elites (1 caster 1 melee) will spawn BEHIND you - go back and kill them before you move forward.

You should be able to Kill Rahk Zor (i 4 boxed him with a VW tanking at 17 just pull him without adds) and maybe Sneed at your level - the foundry (with the goblins) & the pirate ship may be a bit tricky before yr main tank and healer are lv 20ish.

Practice, Practice, Practice