Mario vs Luigi is a highly strategic DS game based on outplaying your opponent. The strategies outlined in this article are for 3 life 10 star games.
Skill Tricks:
Mega mushroom dodging: Using any pit can allow you to dodge a mega mushroom. Wall jumping just low enough to avoid being hit will save your stars and powerups. If done correctly, you can combo the enemy since they would have lost their fire flower.
Fire flower double hit: A good timed fire ball will allow you to combo and get a second star out of your opponent. Hit them with the fire ball and jump on their head.
Unethical Tricks: These are tricks that are sure to annoy your opponent and may make them not want to battle you again.
Pipes: Pipes are an easy way to avoid any danger. By continuously entering a pipe, you are effectively made invincible. Extremely effective on pipes map.
Castle map: Castle map is difficult to win through lives or stars with some basic caution. Picking castle map when you are losing is a easy way to lengthen a match to avoid any more losses.
Item rigging: Having 3 or less stars increases the chances for a player to receive more powerful items from the roulette. Purposefully taking damage and avoiding stars until you can maximize coins collected can allow you to win through the other player dying. This strategy is most effective on plains map.
Modding specific situations: This is a section for both players of modded maps and modders.
Ropes and Doors: Ropes and doors work as expected. Doors can be abused similarly to pipes. A mapper could place a one way door to avoid this. Ropes however can be a way to promote more mini mushroom usage or to be a risk-reward option. Being hit by a fire ball or shell on a rope will knock a player off.
Despawn glitch: Mario vs Luigi can only handle so many objects on the screen at once. This can be easily seen by decreasing the size of the ice map as the bullet bills will eventually cause the blue spring to not appear until the object count clears up. Players can abuse this by either killing enemies or allowing enemies to stack up to purposefully despawn certain elements in a map.
Bad objects: Normal springs will always crash the game. Launching pipes will desync the game.