Find the Troup that use the less rss (powerful) and the one that use the must ress
Find the Troup that you buy with the less amount of saphire and give you the must amount of ress(you make cheap ress)
Find the Troup that you can train that give you the must amount of ress(you make quick ress)
Balur just want ress
You want power
The game want saphire
You pay balur with the Troup rich in ress
He pay you with the Troup not rich in ress but in power
You won saphire by participating to a tournament
...... Etc
You need to do the math yourself but once you done them it become easy.
But remember the BGs xp count in the calcul so you need to kill the must Troup of balur with the less of yours