Grand Court of Sebelkah is killing me today, any tips on how to push through this one?
Firstly, it's worth noting that Grand Court of Sebelkeh is the noob trap of Nightfall. It exists to teach you the importance of damage. Without knowing anything about what's going wrong for you, I suspect you're not killing things as efficiently as you ought to be. If you are, this mission is pretty straightforward, if not, it's tough.
As for advice, I can't be specific without details. Post your builds - a screenshot of your interface with hero panels open will suffice. Hopefully you picked Norgu.
Aside from that, pulling back at the start to allow the margonites to get together in the middle makes things easier than rushing straight in and fighting three smaller groups on three sides. Also make sure you've got your Lightbringer title up as previously mentioned.