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