This achievement can be unlocked n the last level. "Boss 3", it's the first step on getting the achievement for playing pong.
NOTE: It's best to have the knife that kills in one hit equipped here!
NOTE 2: It's also worth noting that dying during any of steps after landing on the island requires you to start the whole sequence from arrival on the island all over again...
Video Guide:
Text Guide:
So at the end, when the helicopter is chasing you, you are required to start climbing the side of the mountain and destroying bricks of two to get higher. When you get to the top, instead of going down the long corridor to the left and getting the black suit, (Normal ending), you want to jump right, and then immediately double jump back left so that you are on top of the mountain, then immediately run left into an invisible wall, and the helicopter will crash, and it will show the normal ending, but you'll still be able to move from up top. You will see prompt from the creator/dev from there...and moment later you will be taken to the pong game, and that achievement.
Continue playing pong until the creator/dev teleports you to a small island, which will have the blue and black ninjas cheering you on, with a door as well.. The Developer/creator will tell you that your friends are cheering you on and ask you to go through the door, DO NOT DO THIS. Instead, wait a bit longer, and the dev/creator will say in text: "Oh look! There's a collectible past the door", as you approach the door, do a double jump over the door, and collect the collectible.
The door will close and the creator/dev will get angry, from here, use a knife on the box and jump on top. He will spawn two lasers to try and kill you with, double jump over the lasers 3 times, and he will eventually despawn the box, quickly run and grab the black suit, and start smoke shifting (RT) through the laser while running to the right, eventually it will come back, do a short boomerang jump off the cliff and back on (may take a bit of practice), to dodge it the second time, then quickly run to the left and smoke shift (RT) again to dodge the final required laser....
At this point, the dev/creator will get angrier and spawn "minions" to fight you, it's best to stand directly in the middle of the map and shoot left killing that enemy, then right, then so on and so forth. There are about 8 - 10 minions that spawn, so be weary of that...
After the minions are dealt with, the final step will be the dev/creator trying to kill you himself, quickly run and grab the blue suit, he will start firing a barrage of missiles at you, what you will need to do is sprint run (RT) to dodge the waves of them. 2 - 3 waves of these happen... after that, you will fall down a hole and eventually reach a ledge with a treasure box on it, the dev/creator states it's not supposed to be there, and asks you not to touch it, but keep using the X button to bash it open and eventually it will open up, and you will have unlocked the secret ending and the achievement.
TL;DR:
1. Get on top of cliff above final corridor holding black suit.
2. Play pong game until teleported.
3. Wait until dev/creator opens door AND states there is a collectible past door.
4. Double jump over door and collect collectible.
5. Jump on top of box and dodge lasers (3 total.).
6. Grab Black suit and evade lasers with smoke dodge (RT) and boomeranging cliffside.
7. Stand in middle of island and fight off "minions" left and right.
8. Grab blue suit and evade lasers with sprint run. (RT)
9. Fall down hole, land on ledge, and beat up (X) treasure box till it opens.
10. Unlock secret ending and achievement.