There are 2 cameos in this one location.
First of all, If you have unlocked Mike then this is much easier. Mike can be found in "Dungeon's and ♥♥♥♥♥♥♥♥♥", but you need guitar guy to get to him, found in "Future ♥♥♥♥♥♥♥♥♥ 2010". Anyway, just after the first checkpoint, you'll see a ladder going down. Above it are 2 insta-kill blocks, jump over them with Mike (or AVGN, but it is MUCH harder) and follow the path to find bossman for an easy achievement. :)
Here is the area to jump over if you are having trouble finding it:










Edit: Here is a video guide by Nico8912, if you would rather watch the entire process:

I have put this cameo first, because this is the first one I run across in a run of the game. The first map you should do is Future ♥♥♥♥♥♥♥♥♥ 2010 to unlock guitar guy, then this stage to unlock Mike. After unlocking Mike, later you will get to a flying part. Instead of jumping on the bird, use mike to break the wall to the left. Walk through the new opening to the left to find the cameo.
Here is a screenshot showing where the wall was:

Cameo 1:
After one of the fights with an axe-throwing knight, you will jump to 2 sets of floating stairs, one going up-right, then one going up-left. At the top of the first set of stairs, select mike and jump over the insta-kill block, pictured here:


Now, just walk down and right (there are no holes in the ground) to find the cameo, but watch out for the fireball.







Cameo 2:
Just after that last cameo, there will be a swinging spike ball to your left. If you jump to the blocks to the left of this spike ball, you will see a wall with a platform on the other side, pictured here:

Use B.S. Man (unlocked in the Beat It and Eat It stage) to double jump around this wall to find the 2nd cameo in this stage.

Cameo 1

When you get to the first falling log, ride it down a little ways and as it is falling jump to the blocks to your right, pictured here:

Now all you have to do is continue right to find the cameo and some goodies,







Cameo 2 (Keith)

At the top of the log section, you will get to a ladder, pictured here:

Simply jump off of this ladder to the left and fall down for the 2nd cameo for this stage.

Cameo 1 (Egorapter)

When you get to the giant cube of ice, where AVGN comments on how the fire should be melting the ice, go to the very top left. Switch to mike, and you can see some secret blocks appear, pictured here:

Follow these up and left to find the first cameo of this stage.








Cameo 2 (Angry Joe)

In the same area as the last cameo, go all the way down and right until you see the giant snowman shooting rolling snowballs. Kill him and continue right, you will see another platform behind a floating wall, pictured here:

Use B.S. Man to double jump around this wall, and find your second cameo in this stage.

Cameo 1 (The Completionist)

After you see the first witch, you will enter an enclosed area with a bunch of jumps around insta-kill blocks. At the very top left of this area, you will be on the top of the enclosed area when you jump up, pictured here:

Using any character, take a leap of faith to the left off of this block, and you will land on some of the semi-invisible pink blocks. Continue left to find the first cameo on this area.






Cameo 2

After one of the green moving block sections, you will come to an area where you go down a few 1x1 falling blocks. You need to stand on the last solo falling block, pictured here:

Jump with Mike (or AVGN, but much harder) from this block over the insta-kill block to your upper right, and then quickly jump up and right again to reach the final cameo!

Edit: These cheats are for the xbox controller, I have not yet figured out how to input them with the keyboard. It seems like "z" on the keyboard would correspond with "a" on the controller, but it does not seem to work. Also, the actual key "a" on the keyboard does not seem to work either.

To get this achievement, simply enter a cheat.
I have found 2 so far, but I haven't really been looking.

The first is obvious, the (in)famous Konami code.
...In case somehow you didn't know, it's up, up, down, down, left, right, left, right, B, A, (start).
This does not actually activate a cheat... you'll see what I mean.
It does get you the achievement though!

The other is "hedgehog mode."
It is simply up, down, left, right, A
This mode makes you have much faster top running speed, but much less control.

If you guys find some, I may add them here if I can.
This isn't meant to be a cheat list, anyway, just an achievement guide :D

Thanks goes to:

Ura, for his super fast characters guide. I would not have found B.S. man so fast without it. (For some reason I checked every stage except Beat it and Eat It before giving up, oh well)

Nico8912, for his youtube video guide contribution for Bossman.

AVGN/Cinemassacre, for being awesome.

Freakzone, Screwattack, and everyone else who worked on and/or published this game.


And onto the other random stuff...

So I've found a few minor bugs.
The first, I dont know if its a bug or what, but if you swing Mike's sword right as you hit the ground, he will swing it a couple times really fast.
The second, if you go to options, then back to title screen, then back to options, you'll see that mysteriously your cameos and ♥♥♥♥♥♥♥♥♥♥ findings have went up, you can keep repeating this to make them keep going up. It will reset back to normal when you enter a game or exit the game.
Finally, there is a speech event bug in Beat It and Eat it (First noticed by Ura, so credit to him).
Right before you go into the boss door, an AVGN speech bubble pops up, but goes away due to loading the boss fight arena. I wonder what was suppossed to be said?

Some tips for the final boss
So, the final boss is hard, and IDK how I'm going to beat him on Hard As Balls mode. (edit: beat him on Hard as Balls! ....with a turbo controller...)
Anyway, You can easily get passed the first two forms without ever taking damage, using guitar guy.
All you do, is first jump to the lowest platform on the right, and fire left. After he shoots his fireball, jump to the lowest platform on the left, and fire to the right. After he shoots his fireball, jump back to the lowest right platform and repeat. You can easily get to the irritable bowels section without taking any damage. On this section, I stay low so I have the most time to dodge feces, and use AVGN's shooting any direction ability to damage him. I have been able to beat him on old school with only guitar guy though, jumping to the highest ledge opposite of him and firing a few times and then going back down to dodge. The last form is by far the hardest. Keep calm, I usually die from panicking and falling off, lol. Stay on the opposite side of the room from him and fire away, and make sure you read where the giant spike ball is going to go.