Shadows of the Damned

This achievement is story related and cannot be missed. You can earn this playing on any difficulty, and it will pop at the conclusion of Act 4, Chapter 1 - The Big Boner.

This entire chapter flips back and forth between the rooftops of a bright demon city, and a darkness realm through which you tavel as a sort of bridge. During the rooftop sections (of which there are 3, each increasing slightly in difficulty) Garcia is rooted to the spot whilst his Johnson becomes the eponymous "Big Boner". You must use the left and right bumpers to switch between 'lanes' and shoot the advancing demons (the same large ones with red headlamp things that you defeat with darkness shortly before in the story). Arrows on the screen will tell you which direction to face, but will only give the general direction (left/right) not specific lanes. Each demon takes a few shots, except when you land a headshot which will kill them in one hit, and this is what you should aim for to be successful. You will know you have landed a headshot when the demons throw their arms up and fall backwards. THIS IS ALSO WHEN YOU SHOULD FOCUS ON GETTING 5 CONSECUTIVE HEADSHOTS FOR THE 'A HOLE IN THE HEAD' ACHIEVEMENT.*

After successfully dealing with a wave of hellmonkeys, Garcia's Johnson will shrink back down to pocket size and you will be able to move on to the next section by approaching the dark portal behind you on the billboard and pressing the button. By entering the portal you will be transported to the familiar darkness realm in which you must advance quickly, collecting hearts to replenish your... breath? stamina? whatever.

Reaching the light door at the end of the path leads to another rooftop section with more advancing demons (triggered by pressing the X button by the payphone, which releases the Big Boner, schwing!). The first and second rooftops aren't too bad, but the third rooftop can be a pain in the Johnson if you aren't quick enough to dispatch the demons, remember to aim for the heads to take them down in one shot if possible. There are more lanes on the final rooftop and the demons sometimes do a weird knuckle-dragging run, so they can sneak up on ya. Either way, it's worth failing at least once to see Garcia get his face used in advertising.

If the rooftop sections give you grief, take a minute and breathe then keep trying, you'll get it eventually. Also remember this is where you should try for 5 consecutive headshots with the Big Boner, if you don't manage to you can reload and try that rooftop again. I would suggest going for the headshots on either the 1st or 2nd rooftop; there are fewer (but enough) enemies here and it'll be less stressful than trying on the last one.

* Just as a side note, I am certain I didn't get 5 consecutive headshots before my achievement popped. You can tell you have made a headshot when the demons throw their arms up and fall backwards, and I know that I had landed a few shoulder shots between headshots. Maybe it was just a glitch, or maybe it is a little lenient, who knows? Judging by the lower ratio compared to the other 5 headshot achievement, I would say there is a chance that hitting somewhere in the vicinity of the head may work, but try and aim true anyway just in case.