I was going to make a big ol' guide about how to manage waves in arenas, but then I noticed there was a wiki page for it. It was outdated a smidge, so I fixed that. Anyways.

I have a trick for arenas. Particularly undead arenas, but it's handy for all of them.

As you can see in the article, arenas are divided up into two sets of waves. You defeat one set's wave, and the next wave will pop up from it. It's very predictable what wave will spawn next and when it will spawn. For the third room of arenas, it looks like this:

SET I: 6 trash mobs -> 4 turrets -> 4 heavies

SET II: 4 elites -> 2 heavies + 2 healers -> 8 turrets

SET I + II -> 12 elites + 2 healers + 2 trash mobs

Here's how to use this information to flawlessly crush this room:

• To destroy greavers, everyone should be in the center of the arena, placing DBB, artillery strikes, launching hexing haze, deploying firestorm, setting up a vortex, putting up barriers or other otherwise prepare for slaughter before the final wave while the final enemy before that is expiring.
• If the final enemy before the final wave is a howlitzer, this will not work. The howlitzers instantly expire and summon the next wave, while at the same time the corpse will act as a missile and crash into you, messing up your bomb charging and possibly shocking you or doing something equally horrible.
• For this reason, you need to make sure the final enemy remaining is not a Howlitzer. How can this be achieved?
• Kill 6 trash mobs and spawn 4 turrets.
• Kill 3 out of 4 turrets, leave one alive. I usually make this the top right turret. You want a top turret to live so it doesn't snipe you from below the camera.
• Take out SET II: 4 elites -> 2 heavies + 2 healers -> 8 turrets.
• NOW kill the last turret from SET I, spawning 4 heavies.
• Kill those. Lumbers in particular have a very long death animation, providing you lots of time to charge bombs.

Another advantage of this method is that you do not have to deal with 4 heavies while under attack from anything else, and this makes everything in general extremely easy to deal with.

tl;dr: Don't kill the top right turret from the 4 turret spawn. Gather in the center of the arena and prepare bombs while lumbers are dying.