Get Gundam
At the Title screen, hold L1 + L2 + L3 and press Start.
Get Hyaku Shiki
At the Title screen, hold R1 + R2 + R3 and press Start.
Harder Difficulty
At the Title screen, hold R1 + L2 + R3 and press Start.
Get Full Member
At the Title screen, hold L1 + R2 + L3 and press Start.