Super Sudden Death and 300% are Special Melee and Special Brawl modes in Super Smash Bros. Melee and Brawl, respectively. They challenge two 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 radically changes 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 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.
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 167%. With Lucario always at 300% damage this property retains its maximum level throughout the match.