Have any character get to Rank 50 on Barrier Magic to get Field Barrier 3. Then, have
that same character get to rank 8 on Black Magic to get Regenerate MP. When you walk
around killing enemies for SP without fighting, you will regenerate the lost MP and 
never run out, effectively giving you an unlimited SP gain. A good area to do this 
trick is in the flying fortress just after Zola leaves the team and goes through the
other warp. In the room just before you meet Szabo are three lasers. When set off, a
flying Mecha Robo appears for each one. Each monster results in a 2 SP gain. Just run
from the save point through the lasers and back to the save point. Continue running 
this path and saving in between when the lasers stop dropping monsters.

When you reach the primitive cube, before you enter the ruins, outside you will find
butterfly monsters. Activate Field Barrier 3. Place your character in the center of 
the field, and tape the Left Analog-stick Up and Right Analog-stick Right. If done 
correctly, you will run around killing endless spawns of these monsters for +2 SP 
each. This is best done with MP Regeneration. You can reach level 99 in a Shadow in
less than an hour. Check regularly in case of skill obtaining, and also if the 
butterflies get in a perfect chasing distance. If they do this, activate the barrier
again and continue. This trick is also best done with Eyepatch or Black Belt.