Here is the full list of all 22 METAGAL achievements.
Die over 20 times in one level
Defeat Final Boss with Warp GAL
Finish the game with all characters
Defeat Final Boss with Buster GAL
Defeat Final Boss with Dash GAL
Complete the game using gal 01
Collect all powerups
Get Rank S on every level
complete all levels in PAR
beat any level in less than 3:40 minutes
Complete the game once
complete fortress 4
complete fortress 3
collect more than 9 gears
complete fortress 2
defeat GAL0X
defeat GAL04
defeat GAL03
defeat GAL02
defeat GAL01
complete tutorial
watch intro without skip