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