Starlit Kart Racing

You will gain this naturally over the many hours of grinding the cups for the tokens for unlocking the carts and cups, but, if you're like me and are just trying to grind this out for a specific target, goal, etc then you can go to Local Versus

Track Style: Short Races
Items: [your choice]
Opponents: Players Only
Laps: 1
Races: 4

Select 300cc.

You do not need to plug in a second controller, and you can do 4 races in about 5 minutes, as opposed to one cup taking around 7-10 minutes. This all counts for all of the 25, 50 and 100 races in 300cc.

I do not think this counts towards the 25 victories in a row achievement, however, you can use it to churn out the star boosters and items achievements, just make sure to use every item you get.

Alternatively, you can add the bots in to play against, but I personally find that extends the time of races (through power ups hitting/etc), but can also be used here and there to change the mundane constant of the 4 races.

Edit: This does in fact work for 25 victories in a row, as I accidentally popped it whilst churning this out.