If you want a quick way to level up robots,here's an idea.
Go to the final destination stages and bring a robot with rain astro crush,the robots you want to level up,and a high level as a back up.
Astro crush is the new overpowered lawn mower,the lawn being everything in your sight,and takes out most of the bosses in 3 hits or less.Keep repeating this process until you see fit.

If anyone else has good grinding methods put them in the comments in case some people feel like astro crush is cheating(it pretty much is honestly)


Note this also gives you a huge amount of zenny,with the turn and overkill bonuses.

AND SCREWS DON'T FORGET SCREWS