Here is the full list of all 12 Omega Five achievements worth 200 gamerscore. It takes around 4-5 hours to unlock all of the achievements on Xbox 360.
Go 30 seconds without swinging Sensei's sword.
Clear Stage 4 in Single Player Arcade Mode.
Clear Stage 3 in Single Player Arcade Mode.
Repel 20 or more enemy bullets using Tempest.
Clear Stage 2 in Single Player Arcade Mode.
Cleared Stage 1 in Single Player Arcade Mode.
Held onto an enemy for 10 seconds or longer with the anchor using Ruby.
Maintained the max. bonus for 30 seconds or more.
Complete Single Player Arcade Mode without continues using Ruby.
Complete Single Player Arcade Mode without continues using Sensei.
Complete Single Player Arcade Mode without continues using Tempest.
Complete Single Player Arcade Mode without continues using R.A.D.