At omnigol run past her in the final room and jump into the hole in the wall, have two or three people in there. One in the corner always, and one/two shooting omni. Corner guy is the reviver, others just keeps shooting her. Stay close to the left side and reviver won't even have to move to get the attacker.

Stay in that hallway/room right before the final boss room. From there send two up at a time with arc weapons(they stay in the room, just on the edge) they will be able to do ~5% damage each time, the third person runs up and revives them. Rinse and repeat as needed. Watch for 2 things: 1.they spawn in the door in the room occasionally 2.knights will occasionally enter the room. If you need more detail let me know! Worked for me x3 so it should work fine for you. Best of luck!

Be patient...