Wall of pain
The wall of pain, often abbreviated to WoP, is a Jigglypuff technique in Super Smash Bros. Melee, which involves barraging the opponent with repeated back aerials, exploiting Jigglypuff's high horizontal air speed to continuously follow the opponent in their knockback trajectory. The Wall of Pain is one of the most powerful edgeguarding techniques in the game, as multiple bairs will often send the opponent too far off stage to recover, while Jigglypuff can almost always make it back safely, due to its aforementioned air speed and high amount of jumps. Even if the wall of pain doesn't manage to KO, it can still rack up significant damage and leave the opponent in a poor position to try and get back on stage.
The wall of pain can technically be performed in all Super Smash Bros. games, but it is most effective in Melee due to the games hitstun and air dodge mechanics.
Overview
Tetsuya and AOB were the earliest prominent users of the technique, and they are largely responsible for its widespread use. The term "wall of pain" was coined by SmashBoards administrator and retired smasher Amorasaki, a longtime friend of AOB.
To initiate the wall of pain, the Jigglypuff player hits the opponent with a back aerial in the direction of the edge of the stage, typically at low to mid percents. After each bair, the Jigglypuff moves and jumps in the direction of the opponent's knockback trajectory and follows up with another bair. The Jigglypuff continues interspacing bairs and jumps until it runs out of jumps, or the opponent has been sent far enough offstage where they cannot recover.
After a wall of pain, the opponent will often be too far offstage to properly recover, resulting in a stock loss. If the opponent does recover, they will have sustained significant damage, and upon reaching the ledge, are subject to more pressure from the Jigglypuff in order to make it safely back onstage.
The wall of pain is not nearly as effective in the other Smash games when compared to Melee. In Smash 64, Jigglypuff's lesser jumping abilities prevent it from continuing the combo for as long, while more effective finishing combos exist due to the game's increased hitstun.
In Brawl, the back aerial has lost its knockback, and the move is no longer a true combo; opponents can simply air dodge to escape the wall of pain, or counterattack with a move of their own. Thus, while the wall of pain can technically still be performed, the technique is no longer a viable edgeguarding maneuver, as Jigglypuff now has other, superior options.
In Smash 4 Jigglypuff's back aerial has much more knockback again and is designed as a finisher rather than a combo move, making it easier to finish a WoP with a back aerial but harder to keep one going. As such, most WoP's in Smash 4 generally utilize Jigglypuff's forward aerial (and neutral aerial as a finisher) instead.
Other characters
While other characters also have midair jumps, they do not have the appropriate recovery potential, with exception of fellow featherweight Mewtwo, the other characters do not have the require air speed, or falling speed in order to fully emulate Jigglypuff's wall of pain and be able to return to the stage safely. Bayonetta, Dark Pit, King Dedede, Kirby, Meta Knight, Mewtwo, and Pit can perform similar techniques, and some other characters, such as Captain Falcon in Melee, may have "pseudo-Wall of Pains" with similar outcomes, which are not based on a back air. These characters lack either the horizontal air speed or multiple jumps, rendering these techniques less effective compared to the original wall of pain in Melee.
Bayonetta: Bayonetta can forward air a few times knocking her and her opponent towards the corner bottom blastline it can be mixed up with witch twist and after burner kick. With Bayonetta having arguably the best recovery in Smash 4 it is likely she will make it back to the stage or Use wall clinging to the jump to the edge.
Dark Pit/Pit: Dark Pit and Pit are very capable of using wall of pain with their coward air and multiply jumps and the Power of Flight gaining huge distance.
King Dedede: King Dedede has many jumps and can use ether forward or back air however back air is much harder to land consistently, depending on the situation King Dedede might be able to meteor smash the opponent as a finisher.
Kirby' Kirby has similar jump to Jigglypuff however Kirby hits thrice for each forward ariel keeping opponents at bay as well as a meteor smash, and sucidial Cutter.
Meta Knight: Meta knight has three hits per foward and back airel making it great for wall of pain backair can be fastfalled for a meteor smash like effect. Meta knight has many jumps allowing him to traverse and with the assistance of Shuttle-loop allows for him to grasp the edge easily.
Mewtwo: Mewtwo like Jigglypuff is a featherweight and with Mewtwo’s long tail allows Mewtwo to reach better. The unique propertiss of his tail depending on where it makes contact with depend on the amount of knockback receive for example the tip does low knockback but high hit stun allowing it to be followed by another, on occasion with the right circumstances Mewtwo can wall of pain very quick and easy due to his air attributes, and having the 2nd highest double jump behind Yoshi, Mewtwo can use confusion in the wall of pain if desired but might fail if the opponent has a jump. Mewtwo sometimes might end up above the oppenent and catch a air dodge with down airel. For recovering Mewtwo’s confusion acts as a horizontal jump and teleport can be used from off screen to an omega form stages if Mewtwo is at the proper height.