I got this achievement after completing the game on Casual without using any green Gel for the 'It Is What It Is' Achievement.
Upon starting New Game+ I had accumulated over 350,000 Green Gel which I then used on various upgrades. The first available time to upgrade your character is in chapter 2 in the cabin mirror.
You should make a save before doing any upgrades and select them in turn. For example you could get Atrributes all done, re-load the checkpoint and do the Guns and then Stock. Then when you're ready to replay the game pick and choose what you want/need.