So it seems that Valmo's discovery with Mirror Gazing and the Anniversary Pocket Mirror makes this perhaps an even better powerleveling place to fight. Capturing all 5 opponents here gives and extra 10 XP from MG with both mirrors equipped. This doesn't appear to be tunable XP though (eg. with a puzzle box form), but 19 to 29 XP per fight might be worth it.
The downside is that you'd have to target each opponent individually instead of nuking them all with an AOE, so I'm losing ~30 HP per fight this way. If you have some good first aid (Healthy Glow, Trauma Tech, Personal Certainty, Surgical Mask?), and a ton of Omnitech bandages (from Little Eddies), you probably wouldn't have to rest /that/ often, but it depends what your budget is.
(And regarding Kinak's comment above, it would make sense for the guards to just shut the doors completely after killing a certain number in a day. Attack too much and now you have to scale a 30 foot wall while being shot at by wall turrets to get in. Or hack the door controls…. Hm.)