Diablo 3 Act II Cooperative Achievements Guide

Achievements for Killing Belial in Co-Op

Killing the Boss Belial gets you an achievement in each difficulty that you kill him:

The Lord of Lies (Cooperative) (10 Points)
Kill Belial on Normal Difficulty in a cooperative game

Also:
Cooperative Nightmare
Cooperative Hell
Cooperative Inferno

Killing Belial on Inferno gives you the following reward:
Banner Sigil (Pentagram)

The Lost Treasure of Khan Dakab (Cooperative) (10 Points)
Complete the Lost Treasure of Khan Dakab event in a cooperative game

The Tomb of Kham Dakab is a dungeon that randomly spawns in Dahlgur Oasis.

Shrine of Rakanishu (Cooperative) (10 Points)
Complete the Prisoners of Kamyr event in a cooperative game

Another cooperative event in Dahlgur oasis, requiring to free some guards that are held captive

Crumbling Vault (Cooperative) (10 Pts)
Complete the Crumbling Vault event in a cooperative game

The Crumbling Vault is a timed dungeon. If you do not reach the treasure vault room before the timer counts down, you will fail the event. It's recommended you head straight for the vault room and not fight monsters unless really needed.

Guardian Spirits (Cooperative) (10 Pts)
Complete the Guardian Spirits event in a co-operative game

In Stinging Winds, find the Dying Necromancer and talk to him, to begin this event.

The Restless Sands (Cooperative) (10 Pts)
Complete the Restless Sands event in a coop game

Starting NPC for this event: Mehtan the Necromancer (Stinging Winds). He is a random spawn.

The Rygnar Idol (Cooperative) (10 Pts)
Complete the Rygnar Idol event in a co-op game

In Stinging Winds, look for a random dungeon called "Chamber of the Lost Idol".

Lair of the Lacuni (Cooperative) (10 Pts)
Complete the Lair of the Lacuni event in a cooperative game

For this one, enter the Abandoned Mineworks in the Howling Plateau

A Miner's Gold (Cooperative) (10 Pts)
Complete the A Miners Gold Event in a cooperative game

Look for an NPC seeking help in the Howling Plateau.