Recycler: Uppercut somebody with a weapon, take their weapon and kill them with it
Kill it with fire: use flamethrower or ray gun
Kapow: Get 3-4 noob CPU players on a flat level (e.g. chaosseum). while the n00bs are fighting, charge up your punch.
Anger management: 6 n00b CPU players. get on one side and keep punching. you will soon have rage and KO all 6 in one shot.
Shopper/Crate Expectations: Deathmatch, 60 crates p/min.
0 Gravity: side swipe + double jump.
Immortal: Tribal trouble or Farmageddon.
Architect: just a level with 1 long platform. last one standing, 1 life. (you have to finish the match to get the achievement)
Veteran: do the above 10 times.
Pacifist: LAVA PALAVER. dont hit anyone and shield half the time. the time bar on the top is how long until you win.
Take the plunge: MANSION MELEE, noob CPU. use doors on the sides to avoid CPU.
Survivor: use your skill! kill at least 10 people to get 10000+.
Up and away: enable rage match and 2x height. hit someone with a Z rage attack.
Lift Off: enable 5x damage that youve just unlocked. keep rage on.
Magic number 3/4/5: enable 3/4/5 noob CPU with only rpg. blow them all up. i think the cpu will always drop rpg for some reason.