1. Make sure you get Attack Boost. This is REALLY important for the grind.
2. Make sure your 2 Robot Masters of choice are at Level 100 like this: Lv. 100. You can use only 1 Robot Master but 2 are recommended as you might lose track of your robot's health and you'll end up defeated.
3. Make sure you have 99 E Tanks. If you don't have 99 E Tanks, you can buy some at Auto's Shop. You can optionally buy 50 extra lives to revive your other Robot Master if your other Robot Master is K.Od.
4. Get to grinding. Use Attack Boost twice to get yourself to 4 ^s. Every time you get a victory, 1 ^ will decrease. If you get 3 ^s, use Attack Boost again to get yourself to 5 ^s. It won't get any higher than 5 ^s, so keep in mind that. If your health turns red, then drink an E Tank to avoid a game over and continue fighting. If you run out of E Tanks, then exit the stage.

EDIT: If you get a Fusion Star Whole, you'll end up in the Stage Select screen.