How to Beat Nexus Core Arena
Buy 8 (or 10) assault rifles, give each AP rounds. Swap them between waves. If you are playing gun tinker origin, just pick any gun and give it AP rounds.
On some areas there are highgrounds which are way easier to defend, like on second wave there is a square on the very end of the map, on third area there is a highground, on first area hellipad seems easier to defend and on the very last area there is plenty of cover on the top-right.
Do not kill dude with shields on stages where MAG agents spawn (GO3EM i think they are called). New enemies spawn when there are like 3 enemies remaining from the last wave. Shield bros are slow and melee, so you can pretty much 1v1 MAG. It would be way harder if last enemies left are red berretes, you will have to kill them, potentially spawning more MAGs.
Note: Switch to silenced weapons in the final 2 stages of the arena and keep ambushing nexus engineers from behind. Their Tac bars dont work when they cant see you.