You get to his room by taking the door directly opposite the entrance from Ssra, turning left (this is the route we take, ymmv) entering the Revenant room then taking the hidden corridor down to his room.
Based upon my experience of the first 2 Rhags, their resistance to elemental and magical nukes is minimal...at 54 and 55 I was landing 100% on Rhag 1 and 2. The Rhags are permarooted and summon, naturally, so you have to clear your way into him.
Our tactic, which so far has proved to be effective, is a MA with Rune and CHeal rotations one one side and all other melee on the other side, at max melee distance to try to minimise the ae effects.
As far as nuker strats go: I found that with the right gear, its possible to chain nuke him and avoid aggro. I opted for a nuke rotation of draught-draught-right click conflag from VRoT-Ice Comet. At the time of posting I was running with no AA enhancements to my nuke dmg...one of our alliance's nukers has Casting Fury and I'm not sure I've seen her survive a Rhag encounter to date :)
Nenelyan
Torv