The best level I found to farm currencies on was The Weapon Forge in the Prologue. Completing the crown races gives you bonuses at the end of the level. If you use Malady you can teleport through some of the traps and if you have her top weapon, a constant minion supply. (I suggest missing a weapon in the event of a patch), It also goes a bit easier if your minions are actually collecting treasure for you - if you're having minion behaviour problems, try a hard reset.
You will need in total:
1000? blue spheres to fully upgrade each minion type (I didn't note the first 4 upgrade costs)
400 weapon shards per Netherghul (golden cubes from chests)
1026? purple spheres to fully upgrade a Netherghul.
The yellow diamonds are effectively useless, as they only go towards minion hats. Guessing about 10k to get them all.