Giga Bowser Punch
Bowser transforms into Giga Bowser, and allows the player to control a targeting reticle and initiate a single powerful punch before the Final Smash ends.
Bowser grows to an enormous size, appears in the stage's background, and takes aim with a player-controlled crosshair similar to that of the Dragoon. The player can unleash a single punch at the targeted area, dealing 40% damage and launching struck opponents; if the opponent's damage percentage is 60% or greater before being punched (80% when using the weaker FS Meter version), they instead receive an instant screen KO.
Like regular screen KOs, only up to three players will be screen KO'd in 8-Player Smash; otherwise, the victim will be sent towards the upper blast line with extremely high knockback (comparable to Jigglypuff when breaking its shield).
In certain stages, when the punch would ordinarily trigger a screen KO, the opponent will instead float upwards towards the upper blast zone, passing through all solid objects, until they are KO'd. This issue does not appear to be caused by screen KOs being disabled, as in training mode (where screen KOs are always disabled), the player is still instantaneously KO'd, and the bug's occurrence is solely determined by stage choice. The following stages are affected:
Giga Bowser has not appeared in any Mario games. Instead, Giga Bowser first appeared in Super Smash Bros. Melee as a hidden boss in Adventure Mode and as an opponent in the final event, Event 51: The Showdown. To fight him in Adventure Mode, the player must complete Adventure Mode on Normal difficulty or higher, in 18 minutes or less, without using any continues.
In Brawl and SSB4, Giga Bowser is Bowser's Final Smash rather than a boss. In Ultimate, like other "transformation" Final Smashes (such as Giga Mac and Wario-Man), it has been changed into a short powerful attack, where the character transforms into the powerful form only briefly, promptly returning to normal gameplay.
Giga Bowser is likely based on the tradition of final boss fights in Mario games involving Bowser growing to gigantic size; this first appeared in Super Mario World 2: Yoshi's Island (featuring Baby Bowser). Giga Bowser's presence in the background and increase in size are directly compared to Yoshi's Island, and the subsequent punch is similar to attacks performed in the final Bowser fights in Super Mario Galaxy 2 and New Super Mario Bros 2.
Bowser also had the ability to grow giant and punch in Mario & Luigi: Bowser's Inside Story, though the perspective was on his side, rather than his front.