I've got to clean out some box space so going to list some monsters and see if anyone can come up with a reason to hang on to them. Mostly it'd be for low cost / technical / special circumstances...
BAO Catwoman multiples
BAOFirefly (I have no Hino to care about skill ups, and have Angling in case)
Rowdy Red Samurai (already have two max skill Titan)
Mid Fire Ninja
Low Fire Ninja
BAO Grundy (have other resolve options, but only one Grundy... May as well keep)
Neneko (have no blue ninja for the skill ups and only a couple of these)
Low Water Ninja
Fafnir (this one is a plus egg which I think is more valuable than the card itself)
Dragonettes (again plus eggs that may be worth more to feed)
Mid Wood Ninja
Low Wood Ninjas
GC Crab
ECO Tinies
AB Yellow Bird Chuck (keep for other Call skill ups?)
Shugen Demon, Shouken
Low Light Ninja
ECO Dumpty
Shugen Demon, Jyaki
Probably can feed / sell / delete most without trouble. But last time I sold things that would have been helpful on Tengu and the other conditional.
There really is no reason for me to like this stupid game as much as I do. And the sad thing is I like the inventory management, skill up, evolving, REM stuff more than the actual puzzle part.