Here's the fastest method for farming MP Points in 100%.

Your best bet is to play Party Mode (Battle Royale) on DK's Stone Statue with 30 Turns. Set the CPUs' initial difficulty to Easy, and give everyone (including yourself) a 3-Star handicap.

For most of the match, your focus should be gathering as many Coins as possible. Most of the time, you'll want to take the right-hand path to try to get Coins from the bees. You'll want to take at least one lap through the Item Shop, buy exactly one Star Pipe, and save it for the endgame.

When the opportunity to purchase a Star presents itself, DECLINE IT! Trust me on this. This will ensure that you stay in last for most of the game, which makes you a bigger target for Friend Spaces and a smaller target for Duels.

When the Final 5 Frenzy comes around, ensure that the wheel lands on "Get Stars for 5 Coins". If it doesn't, you can reset the turn and try again. The next time you reach the Star Splurge Space, buy every single Star you can afford at a quarter of the price - which should ideally be at least 50.

On the last turn, set all the CPUs to Expert, and use your Star Pipe to make any last-minute Star purchases. If done right, you should earn around 1500-2000 MP Points per game.

Mod Note: According to DJ_Tala playing Mario's Puzzle Party is slightly faster than this method. You can get about 5000 points per hour doing so. Happy grinding!