Court of Oryx has a third tier you can access by getting an Antiquated Rune. Get it by reaching rank 5 with Eris, and charge it by completing a Stolen Rune fight three times. In the third tier you will have to face a new boss called Krogoor, a Wizard. It starts just like any other CoO fight. You drop down to the Court, the rune placer walks up to the portal, and the fight is on.
Krogoor will spawn on the top platform, but will have an invincible shield on her (the white one). A huge ogre will spawn directly in front of Krogoor on the top platform and will stay up there. In addition to the ogre, five eyes will spawn. Killing one of them will give you an ogre buff, which is x2 damage on ogres.
Once the giant ogre on top is killed, you can then start doing damage to Krogoor. You only have about 20-30 seconds though before another giant ogre will spawn and she becomes invincible again.
Of course, you will need a great firepower and also positioning your team in a proper manner – by the first two rocks closest to the wall will be fine - to defeat this boss. Another good spot is on the left and right edges, there is kind of a head-glitch half way that you can hide behind.
Everyone should pick off one of the eyes and start putting shots on the big ogre. About 10 seconds in, one small ogre will spawn. Direct your gunfire towards him and take him out fast, you can also avoid using super as its head is huge and you won't need them.
Once he is dead, make a B-line for the top platform. Three ogres will immediately spawn on the ground level, but ignore them, they can't shoot you when you're up on the top platform as they don't have a good line of sight. Once everyone gets up there, everyone pop their supers and just put as much DPS on Krogoor as you can. If you don't manage to kill her there, then right when you see her shield come back up you need to run back to the two rocks because a massive ogre is going to spawn and he'll immediately smash the ground and kill everyone.
Remember that dying or wiping (where the rune placer cancels the event in his ghost menu) does not take your antiquated rune. The rune placer will have a chance to get legendary stuff like artifacts, others will gain a good amount of reputation points.