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