AereA

After unlocking "Legato", you will then need to max out the skills you just got to Level 3 with your Clefines. These skills can go past Level 3, but make sure you don't do that, as it is a huge waste of money (and therefore time).

The best place to grind money is the first snow mountain level, as it has the most pots, and the least amount of backtracking and areas without pots (unlike the second snow level which is much larger and confusing). Simply do a run, and then you can either take the teleporter back to the concert hall if you're near it, or just quit to the main menu. The game saves automatically, and when you load it back up, you will always spawn in the concert hall.

I would suggest grinding this in small increments. Each character takes about 25k to fully upgrade (the mage a bit more, the warrior a bit less), but instead of simply grinding out all 25k at once, set smaller goals and buy each upgrade as you can. It will make the grind feel a bit less daunting, despite taking the same amount of time either way.