Successfully complete story mode to unlock the Infinite bonus level.
Defeat Colossus (Sector 2 Base Boss) to get the Inferno weapon.
Defeat Enforcer (Sector 2 City Boss) to get the Judgment weapon.
Defeat the Infinity level 30 Boss to get the Neutron Gun weapon.
Defeat the Infinity level 10 Boss to get the Onslaught weapon.
Defeat the Infinity level 60 Boss to get the Trident weapon.
Defeat the Infinity level 20 Boss to get the Vulcan weapon.
Defeat the Infinity level 50 Boss to get the Blast Helm armor.
Defeat the Infinity level 40 Boss to get the Surge Helm armor.
Start to reload then switch to a different weapon. Switch back and your weapon will be reloaded.
The weak spots for the three Bosses in Sector 2 are as follows:
Colossus (Base Boss): Climb the stairs until at the very top and look down at Colossus's head. When the head opens, shoot it with a shotgun for best results (1/5 to 1/4 damage).
Enforcer (City Boss): On the machine's back near the bottom, is a circular lighted area. Your crosshairs will lock on to that area. Shoot it with a shotgun or rocket launcher for best results.
Mantis (Ruins Boss): Get close to the bug and shoot the underside. Shooting its shell will not do anything, as it deflects most of the shots. The best weapon is the Photon MG because it stops Mantis in its tracks.