Boss #1: Hans Grosse

Stand far away from him and unload on him with you chain gun.

Attack: Dual Chain Guns
Approximate Shots to Kill: 35 Bullets or 5 Rockets
Item Dropped: Gold Key

Boss #2: Dr. Schabbs

Stand a fair distance away from him and strafe while you shoot to avoid the syringes.

Attack: Throws exploding syringes
Approximate Shots to Kill: 50 Bullets or 8 Rockets
Item Dropped: Gold Key

Boss #3: Trans Grosse

Not much tougher than Hans Grosse, so use the same stratagey.

Attack: Dual Chain Guns
Approximate Shots to Kill: 45 Bullets or 7 Rockets
Item Dropped: Gold Key

Boss #4: Ubermutant

Larger, deadlier version of the regular mutants. Standing far away and using a chain gun should work again.

Attack: Chest-mounted Chain Gun
Approximate Shots to Kill: 55 Bullets or 9 Rockets
Item Dropped: Gold Key

Boss #5: Death Knight

Stand far away from him and unload on him with you chain gun while strafing to avoid his rockets.

Attack: Dual Chain Guns and Dual Rocket Launchers
Approximate Shots to Kill: 60 Bullets or 10 Rockets
Item Dropped: Silver Key

Boss #6: Mecha-Hitler & Hitler

Hitler starts off in a robotic suit and then appears in his uniform when you destroy the robotic suit. Not too tough, just use the usual stratagy of standing far away.

Mecha-Hitler
Attack: Quad Chain Guns
Approximate Shots to Kill: 40 Bullets or 6 Rockets
Item Dropped: None

Regular Hitler
Attack: Dual Chain Guns
Approximate Shots to Kill: 30 Bullets or 4 Rockets
Item Dropped: None