Downwell

You collect gems as you kill enemies, inside caverns (while not using Armspin style), blocks (the ones with a gem encrusted on it), or building a 8+ hit combo (each combo gives you 100 gems). You need to have 5000 gems unspent with you at once, so you must avoid buying anything until you get the trophy.

The best way to get this one is selecting Levitate style, as it will give you time to kill enemies, and help you avoid landing while building your combo. Do it on Normal, as Hard Mode makes it almost impossible to not spend gems (unless, of course, you are ungodly good at this game). Start the game and around Area 4-2 you should get enough gems to get the trophy (while also getting Saving Up)