Borderlands 2 - Went Five Rounds

Complete Round 5 of any Circle of Slaughter - 25

There are 3 Circle of Slaughter Locations in the game (not including DLC)

Bandit Slaughter
Starting in The Fridge, travel to Fink's Slaughterhouse, this Circle of Slaughter is run by Fink. You will get the Hail Assault Rifle after completion. You will fight Rats, Psychos, Marauders and Nomads. This is the easiest of the Circle of Slaughters because of the lower levels.

Creature Slaughter
Starting in the Wildlife Exploitation Preserve, travel to the Natural Selection Annex, this Circle of Slaughter is run by Captain Cabrera. You will get the Creamer Rocket Launcher after completion. You will fight Skags, Spiderants, Varkids, Bullymongs, Stalkers and Threshers. This is Circle of Slaughter is of medium difficulty.

Hyperion Slaughter
Starting Eridium Blight, travel to the Ore Chasm, near Mal, the friendly Robot. The Hyperion Circle of Slaughter is run by Innuendobot 5000. You will get the Chere-aime Sniper Rifle after completion. In this Circle of Slaughter, you will fighter Loader Bots, Constructors and Hyperion Fighters. This is the hardest of all the Circle of Slaughter because of the higher levels.

After completing all five rounds of any of the Circle of Slaughters you will get the achievements.

Fink

Captain Cabrera

Innuendobot 5000