This is a campaign achievement.
Once you have reached the top of the tower in the plaza on Genosha, the ball will fall and Deadpool will meet Cable again. Cable will ask for two Sentinel parts to reconfigure the Sentinel boot to Deadpool's new "specifications".
The first part is behind the nearby door, which you can open when prompted by pressing LB. use the Sentinel arms to let Deadpool jump up to the ledge where the part is.
The second part is through the underpass on the other side of the plaza. Another Sentinel arm is here, used to shoot the walls to gain access to the part and then to destroy waves of enemies.
Once both parts are given to Cable, the boot will fly into the air. Deadpool will use the cannon to fight off enemies, with the boot randomly flying into the air again. A mid-air collision with a certain character will lead to the next section and the achievement will pop.