My strategy varies depending on what characters I'm playing, so here we go:
Light (Ex: Greninja, Fox, ZSS) - Try to find challenge doors as quick as possible. After, run around and defeat medium-sized enemies. Around 1 minute left, find the nearest player to me using the map and KO them. Be sure to grab as many Special, Strength, and Defense powerups as possible.
Balance (Ex: Mario, Lucario, Pac-Man) - Use items to defeat large enemies, and don't approach Challenge Doors. Be sure to grab as many Arms, Defense, and Strength powerups as possible. If there happens to be another player nearby, KO them.
Heavy (Ex: Bowser, DK, King Dedede) - Defeat large enemies without using items, and then take on Challenge Doors. Be sure to grab as many Defense, Speed, and Strength powerups as possible. If there happens to be another player nearby, KO them.
Special (Ex: Mega Man, Robin, Palutena) - Try to find challenge doors immediately. Run around and grab as many chests as possible. Then, defeat larger, more singled out enemies. Be sure to grab as many Special, Speed, and Agility Powerups as possible. Try to KO another player as soon as you think you are buffed up enough.
Swords (Ex: Link, Ike, Marth) - Try to find challenge doors immediately. Then, defeat hordes of medium/small enemies. Try to grab chests if possible. Be sure to grab as many Special and Strength powerups as possible. Try to KO another player as soon as you think you are buffed up enough.
As for characters I'll be playing, It'll probably just be random.