Butchered! (Cooperative) (10 Points)
Kill the Butcher on Normal Difficulty on Cooperative Mode
Also more achievements for beating Butcher on Cooperative Nightmare, Cooperative Hell, and Cooperative Inferno.
After beating the Butcher in Inferno Coop mode, you get as a reward a Banner Sigil (Gryphon)

The Butcher is found in the Chamber of Suffering, towards the End of Act I, in the quest "The Imprisoned Angel".

Revenge of Gharbad (Cooperative) (10 Points)
Complete the Revenge of Gharbad event in a cooperative game

This event can be found in the Southern Highlands (Ancient Pyre)

Jar of Souls (Cooperative) (10 Pts)
Complete the Jar of Souls even in a cooperative game

This event happens in one of the fake tombs under the Cemetery of the Forsaken.

The Matriarch's Bones (Cooperative) (10 Pts)
Complete the Matriarch's Bones event in a cooperative game

Another event happening in one of the distraction tombs under the Forsaken Cemetery

Eternal War (Cooperative) (10 Pts)
Complete the Eternal War event in a Co-operative game

Search for a Nephalem Monument in the Festering Woods for this one

Last Stand of the Ancients (Cooperative) (10 Points)
Complete the Last Stand of the Ancients event in a coop game

Another event in Festering Woods, climb a flight of stairs to interact with a memorial stone, then defeat the invading enemies to get this one.

Carrion Farm (Cooperative) (10 Points)
Complete the Carrion Farm event in a co-op game

Look for this event in Forsaken Grounds