This is a guide dedicated to getting easy level-ups and of course, getting one of the last few achievements that will probably be on any completionist's list, Mightiest in Gensokyo, whose obtaining is entirely dependent on you reaching level 99 with either Remilia Scarlet or Sakuya Izayoi.
It works with the assumption that you've already reached Patchouli Knowledge in the dungeon's B10 level after the supposed "final boss" and you have the ability to directly select her stage on the World Map, and you want to get every other achievement before you finish the game completely and get the last two achievements (provided you've Bullet Hell Mode turned on), or you just don't care for the order of doing things as long as they get done.
A Basic Explanation
The basics are simple: Taking advantage of how the game hands out experience, and when it does. Upon the defeating of any boss character you're given your XP and of course any level-up that you had coming.
The method for quickly leveling-up with Patchy is pretty much just this, except when you defeat her and the usual fanfare of beating a boss character happens, press the Start button to confirm your experience gains (which once you've depleted Patchy's health gauge you will have undoubtedly gotten) and exit the level by selecting "Back to World Map."
Once you're back to the world map, simply go back to Patchouli's stage, engage her again and repeat the process. Even at Level 78 the gains are substantial enough to make this an efficient means of closing in on those last few levels with relative ease, and with relative quickness.
Past 60, and given good enough equipment, you should be able to just go at her relentlessly. Let her have it.