Super Sudden Death and 300% are Special Melee, Special Brawl and Special Smash modes in Super Smash Bros. Melee, Brawl, and Super Smash Bros. for Nintendo 3DS/Wii U, respectively. They challenge two to four (eight in Nintemdo 3DS/Wii U) players to a match where each has a 300% Damage Percentage from the very beginning, as opposed to 0%. This is much like the Sudden Death mode which occurs after a tie in all of the Super Smash Bros. games, with the only difference from being that this mode lacks the falling Bob-ombs.
Super Sudden Death and 300% radically change the way the game is played as almost any attack with knockback can kill the opponent. As such, this mode tends to greatly favor extremely fast characters such as Sonic and characters with powerful projectiles such as Samus. Even Pit's arrows can cause significant knockback in this mode. Conversely, slow, powerful characters such as Ganondorf are at the greatest disadvantage because the advantage they gain through greater knockback from attacks is negated by the fact that all attacks have extreme knockback. However, both players are able to tip the scales either way in their favor. Also, items are prone to end matches very effectively. Notably that it's possible to reduce the Damage Percentage to 0%.
Lucario tends to have a greater advantage in Super Sudden Death mode as its aura-based attacks become more powerful as it takes damage, capping at 182%. With Lucario always at 300% damage, this property retains its maximum level throughout the match unless Lucario consumes large amounts of healing items.
In Super Smash Bros. Melee, the songs on Mushroom Kingdom and Mushroom Kingdom II are changed to Mushroom Kingdom (Finale) and Mushroom Kingdom II (Finale), respectively, during this mode (if one does not play the Dr. Mario alternate music), as they are in the regular Sudden Death.