Shadows of the Damned

5-4 is the last boss fight. You'll need to use all the different guns to kill him, so upgrade appropriately.

At first, just shoot him with whatever you want, anywhere on his body counts (be careful not to shoot Paula when he "flashes" you). Eventually, he'll get stunned and kinda hunch down. Use your hot boner's charge shot to put an orb on his chest and blow it up.
He then splits into 3 body chunks floating around. Use the blue machine gun to lock onto the glowing body chunk and blast it.

Once he reassembles, he'll form a green shield. Use your green skull gun charge shot to lob a bomb into one of the holes. Dodge if he's about to shoot at you; there's no time limit. After that, he'll also start creating Darkness, which can be extinguished from a swinging goat head in the middle of the ceiling. Keep an eye on Flemming while aiming at it, he keeps attacking in the Darkness.
After you get in enough shots between the Dark and the beam attacks, he'll slouch over again. Hit him with another sticky payload from the Hot Boner (lol) and shoot the glowy body chunk with the blue gun again.

Same process this time, but he does do a new attack from now on where he opens his chest and a black hand grabs at you. This CAN be dodged. If it grabs you, you go to a darkness level where there are 3 doors. You have to use the Hot Boner to blow them up until you find one that glows white and you can escape. Keep shooting Flemming like before. He should eventually break apart and you can shoot him up one last time with the blue gun.
After that, he'll die for good and BAM. Achievement.

EDIT:
According to ShadowXBL, there's a glitch in the boss fight on Hard mode:
The last round his crack doesn't show on Hard. You have to get close and keep shooting him until it appears.

Here's a video: