Bringing the power of the bows down? Why? The way people were getting this far is already fixed by changing the mob's spawn pattern. With the changes you implemented now you:

1. Made archer an unviable class. Less power on bows means no chance at all to survive, with the risk now that mobs spawn right in front of your nose you won't be able to kill them before they  give you at least 1-2 hits. Aka, you'll die in a few waves.
2. Made any class which depends on keeping distance from the mobs unviable, since a charged creeper can spawn in your back and boom, you're dead. Only tank will be a usable class now.
3. By adding a minimum amount of players to start (read minimal 4) you've ruined the activity of the mob arena. Whenever I'm online, we always go in with 2 or 3 players. There are hardly even 4 people on at my online times to start the arena with.

I'm happy that you changed the spawning mechanism of the waves, but please, for the love of god, make that the ONLY change to the mob arena, and keep the rest as it was.

1.) Archers have still made it farther than the tanks due to them holding back. The "randomness" means they won't always spawn right next to you on their spawning areas, they'll spawn all over the arena, but only on their spawning areas.
2.) I played Healer and lived to about wave 10. (Only because we had one melee class, who did everything while archers stayed back, as usual)
3.) We haven't added that in.

Anyway, I made it to Round 30 as Tank. Only reason I didn't go farther was that the boss glitched. (Endermen)

Highest round: 51
Class: Tank

group of about 4 or 5 at the end