If say, the swarm turrets dealt no damage and just stunned players who get hit by the bullets, that would be cool. I'd actually play LD again just to go to that map.

Also, add a few hard to reach passages that lead you from one side of the map to the other, and where the swarm seed is, a large open plain with ruins or stone blocks for cover from gunners. In all, I like Dusk's idea.

EDIT: The cross map passages could have one way gates, so say, when one side is entered, the door entered closes behind the player until they get to the other side.

Monsters in the arena would be a hard thing to balance. For one, if you had them there, they'd have to deal no damage, and being what you suggest, which is a void mecha knight, they'd have to have no shock effect. On doing that, the mecha would just become a nuisance to players, and it would be pointless to have them their in the arena spawning. A swarm seed (or two swarm seeds, in the case of a larger map) would make more sense, as players could either work to get rid of it, so they aren't slowed down, or just spam Striker classes dash to get through it.

Another thought on this, having the swarm turrets dealing stun at random points on the map would be fun, but maybe have a way to disable them, say, a button with shadow particles covering it.