Abuse the Ooze

Just got this one complete, thought I'd offer a strategy. This is a slow strategy, but works well if you get overwhelmed by the damage the Iron Sludges can do.

He starts spawning sludges at 50%. For 100-50%, just burn him as normal, while killing his adds.

When he hits ~40%, break off - don't bring him too low. If you have any way to lower his damage (Curse of Weakness, Thunder Clap, Demo Shout), keep it up on him.

Stick one DPS on each side. On one side, kill all the sludges (they have only a few thousand HP). On the other side, allow 2 sludges to move to the boss, and kill the rest. The 2 sludges will get to the middle, dance around each other for about 5 seconds, then merge into an Iron Slime. Quickly kill it, then allow 2 more sludges to get to the middle from the same side.

Repeat till you've killed 5, then burn all remaining little sludges and burn the boss.

I just did this on my lvl 100 (ilvl 718) MM hunter. I simply unequipped everything and punched the boss until he went under 50%. Then I stopped and waited for enough oozes to spawn and transform. They died with a simple punch. After 5 or more died I killed the boss and got the achievement. Hope this can be of help for other desperate lvl 100s!

DPS him until the oozes come out, then stop dps, clear out any remaining adds, and after a while the oozes start to form into Iron Sludges, which you need to kill, once you kill enough, you can finish him off to get your achievement