Here is how you should build your Vegito for the grind. You don't need to have God Ki, but it's helpful in case you somehow mess up your shot and need to try again.

Here's how you should build an optimal 5 second punching bag for your abusive need of mythical balls.
Kid Gohan is already among the characters with the least amount of health, but with these ability items it should ensure that he has so little HP that pretty much any character could probably take him down in a technique or two.

• Start the match
  
• Skip the dialogue instantly, but keep your fingers on the buttons/keys to use God Kamehameha

Vegito will charge up the attack, but Gohan should just stand there and try to use a basic ki attack. Sometimes he moves in the air, but it won't matter as either way Vegito finishes charging up the attack and it'll instantly defeat Gohan.

• Make sure the reflect ability items option is toggled on or they won't be applied.
  
  
• I only used the Cell Games arena, so it could work worse or better in others.
  
  
• Be patient. The Dragon Balls appear at random between both sets, so you could end up with around 13 Earth Dragon Balls (they don't stack) before even completing the Namekian set.
  
  
• I just took the first image result off google for super saiyan 5 Vegito because effort is hard.