Here is the full list of all 25 Rival Megagun achievements.
Transform into your Mega Gunship 100 times.
Achieve Buddy 2.0's Hero Ending in Arcade mode.
Break Harvester Core's shell using Mega Gunship.
Defeat the Host in arcade mode with no retries.
Clear the portal stage in arcade mode with no retries.
Complete Arcade mode on Easy with no retries.
Complete Arcade mode on Normal with no retries.
Complete Arcade mode on Hard with no retries.
Complete Arcade mode on Very Hard with no retries.
Achieve Gen's Hero Ending in Arcade mode.
Achieve Smith's Hero Ending in Arcade mode.
Achieve Ruby's Hero Ending in Arcade mode.
Achieve Nano's Hero Ending in Arcade mode.
Achieve Dr. Magic's Hero Ending in Arcade mode.
Achieve the Host's Hero Ending in Arcade mode.
Complete the tutorial.
Play 100 versus matches.
Play 500 versus matches.
Play 1000 versus matches.
Fire 500 special attacks.
Simultaneously launch an attack charged to 200%, transform into your Mega Gunship, and spawn at least 25 drones.
Equip 2 gears.
Equip a special weapon.
Collect all cards.
Chain 100 enemies.