Here is the full list of all 20 Just Cause 2: Multiplayer Mod achievements. It takes around 200 hours to unlock all of the achievements on Windows.
Play for a month
Kill 3,000 players by any means possible
Play for a week
Play for a day
Be on the same server as a JC2-MP developer
Kill 100 players at any offshore rig
Play with 5 friends on the same server
Cruise around with four other players in a MV880 with a mounted gun
Drive a taxi cab with 3 passengers for 10km in a single trip
Kill any member of the JC2-MP team (including developers and beta testers)
Meet 5 other players at Hantu Island
Parachute with 5 other players at the same time
Visit the Mile High Club with 20 other players
Visit the undiscovered settlements on the PDA map
Reach the upper limit of the sky
Play for an hour
Kill 10 players at Awan Sendawan Power Plant
Kill five players who are standing on top of a moving plane
Stand on top of a plane in flight for a minute
Connect to a server for the first time