Acquired in Infinity:
Surge Helm: Defete lvl 40 boss - Mantis
Blast Helm: Defeate lvl 50 boss - Enforcer
After beating the game (finishing story mode), the Infinity Sector is opened up. This allows you to play an "infinity" of floors - each is randomly generated.
Judgement: Beat Sector 2 City boss - Enforcer
Copperhead: Beat Sector 2 Ruins Boss - Mantis
Inferno Beat Sector 2 Base boss: Colossus
Onslaught: Beat the boss on Infinity mode level 10
Vulcan: Beat the boss on Infinity mode level 20
Neutron Gun: Beat the boss on Infinity mode level 30
Trident: Beat the boss on Infinity mode level 60