Diablo 3 Act III and IV Coop Achievements Guide

Achievements for Killing Azmodan

Defeating Azmodan gives you an achievement, and another achievement in each difficulty you defeat him in. Beating Azmodan in the Inferno difficulty mode also rewards you with a sigil: Lord of Hell

Each of these achievements is worth 10 points:
The Lord of Sin (Cooperative) - Kill Azmodan on Normal difficulty in a cooperative game.
Cooperative Nightmare
Cooperative Hell
Cooperative Inferno

Azmodan is at the end of Act III.

Crazy Climber (Cooperative) (10 Points)
Complete the Crazy Climber event in a cooperative game
(can be any difficulty)

Search for this event in Rakkis Crossing

Blaze of Glory (Cooperative) (10 Pts)
Complete the Blaze of Glory event in a cooperative game

Starting NPC for this event: Captain Haile (Rakkis Crossing)

Tide of Battle (Cooperative) (10)
Complete the Tide of Battle event in a co-op game

This event occurs in the battlefield and involves slaying some monsters.

Triage (Cooperative) (10)
Complete the Triage event in a co-operative game

Another random event to be found in The Battlefields

Waiting for Reinforcements (Cooperative) (10)
Complete the Waiting for Reinforcements event in a coop game

Start NPC: Captain Halle
Start Zone: Stonefront

Forged in Battle (Cooperative) (10 Points)
Complete the Forged in Battle event in a cooperative game

Start NPC: Ironsmith Maldonado
Start Zone: The Keep Depths

Blood Ties (Cooperative) (10 Points)
Complete the Blood Ties event in a cooperative game

For this event, look in The Battlefields for a random dungeon called "The Barracks". The event can be found inside that dungeon.

Cooperative Achievements for Killing Diablo (Act IV)

Killing Diablo on each difficulty level gives you an achievement worth 10 points:

The Prime Evil (Cooperative) (10 Points)
Kill Diablo on Normal Difficulty in a Cooperative Game
Cooperative Nightmare
Cooperative Hell
Cooperative Inferno

After killing Diablo on Inferno Cooperative, you get the following achievement reward:
Banner Sigil - Eye of Anu

You fight Diablo at the Crystal Arch at the final part of the game.