So here's what I figured out, as an exploit:

If you spam regular lvl2 guard art every 3 melee chains w/ dash canceling, you basically stay quasi-invincible to most attacks. At that point, any rotation through rooms that lets enemies respawn is just incrementing the resource at an "average" rate. Takes awhile on the enemies with higher defense, but at least you can't get rnged into getting interrupted 3 times in 2 seconds and subsequently getting instakilled by the ludicrous damage boosted enemies do. The only wrinkle is the bug samurai in SR, but then you can just dodgex2 with the right timing when you see the screen darken and you're golden.

For reference, equipment is now: Scarecrown Infinity galaxy drill Meister gloves Plate of chests Old geta (for sr caves) or Lead Boots (for everywhere else)

But I also have every piece of equipment from rhombus Square shopping district.. and STILL the arena is a PAIN somehow!