I remember reading that BRU3 can be a wiser choice than BER for the arena, since you usually proc your berserk after killing the enemy you're facing, which makes it pretty useless.
I'm not sure I ever entered the arena on N! (or that I tried N! AoB outside of the funny&imbalanced dual angel challenges), so I can't help much, but yep, you need luck on medkits and/or spawns.
I don't think mob respawn is the biggest problems, since you got cacos to clean up the place, since the challenge dones't let you to much choice : you gotta ruuuush in, which doesn't takes too much time with our melee damages.
Anyway, quite crazy badge, but many diamonds are !
I don't think it's the hardest, since the requirements are probably more luck than just skill.