The fastest way to grind out Portal Master levels is to replay (repeatedly
) the Spell Punk Library. Not the books themselves unless you enjoy them... but specifically the Live Wire Lock that's hiding behind a bookcase by Flynn. This loop is also a great way to get experience, gold and gearbits as there are several hidey holes with items in them. If you don't want to level up your army further, just hit that lock, get 3 sparks and restart the level again.
Immediately from the start - after dismissing Cali's suggestion head to the right and push a bookcase to snag either a gem worth ~100 or a wish stone. Seems about 50/50. Then head up to where Hugo is for another wish stone or gem on top of the bookcases. Behind and to the left are more bookcases with gold on them. Keep moving to the left in the back by Mags and you will see a bookcase to push. You will find a chest, box or pinata here. From that point, move toward the front of the area past the upgrade fairy and find gold by the Skystones board and another hidey spot across from Magical Upgrade Land. The final hidden spot is found by using the bouncy chair in the center of the area and landing on the elevated platform slightly to the left of center. Once you've hit all these spots - go for that Live Wire Lock and then repeat the cycle.
Make sure you set your Portal Master Level 22 and 23 powers to Locks so you can grind more stardust for each Lock completed. If you want to edit powers, hit start, navigate to Portal Master then Powers.