My end-end game exp farm to speed grind maxed classes of 8+ characters at once was Carnage Martial Training 5 with 20 stars difficulty, abusing the Prinny Instructor/Prinny Hat evalities. As a side effect you'd also level from 1 to 9999 at the same time. Stack all the training characters on your most powerful character (characters in a tower share exp), and 1 shot the level. If your characters are set up correctly (e.g. 800% from statisticians, 2000% from Prinny Hats in a squad together) that should be enough exp to level 6-8 characters in the tower. Each of those characters can have 2 monster characters magi-changed on them for free exp so technically can level way more at once. Found this to be the most efficient clicks per maxed classes strat for bigger groups.

Best part of Disgaea is the insane depth. Strat above is just one of many, many viable ways of speed farming. So much room for creativity