Defeat the indicated Boss to receive the corresponding item:

Blast Helm: Defeat Enforcer.
Inferno: Defeat Colossus.
Judgement: Defeat Enforcer.
Neutron Gun: Defeat the level 30 Boss.
Onslaught: Defeat the level 10 Boss.
Surge Helm: Defeat Mantin.
Trident: Defeat the level 60 Boss.
Vulcan: Defeat the level 20 Boss.