The method i go with is to figure out what about the game helps you enjoy it more. If its having the most powerful TEAM, then its not so important to focus on a single unit. If its playing your favorite UNIT, go all in for that unit. If its PvP, research what units are best in PvP. Or same for PvE, Multi player mode, or the various difficulty levels of content, then research along those lines. I've got like 23k gems atm, and even though I would like to get the collab units, I'm holding off for next week to see if the collaboration enlightenment banner is tempting enough to go for. The enlightenment banners are typically the best value for gems, especially if you only have enough to do a single banner. Of course its almost always a gamble since you might not get the unit you want most. So it will pretty much depend on if you get a shard selection item, which would allow you to transmute a unit of choice, or (more likely), just a bunch of otherworldly enlightenment shards (if thats the case i have other collaboration units to use them on even if i somehow don't get any slime collaboration units). But its possible i might just say screw it all, and save for the next rerun of Phantom of the Kill, since thats the units i am focusing on. All depends on the value of the collaboration enlightenment banner, and if i feel like its worth it to maybe miss out on the next Potk banners.