Mad Max

This achievement unlocks upon purchasing all upgrades in the 'Max' tab from the pause menu. This is where you'll upgrade your attack strength and defense, buy skills, etc. It will cost somewhere around 10,000 scrap to buy all the upgrades, which may seem like a lot, but this will take place over the course of your entire playthrough. New upgrades are only unlocked upon progressing further in the story, so you'll unavoidably get a lot more scrap by the time more upgrades become available.

In general, you should make these Max upgrades your top priority as they become available. Always keep a stock of a solid 500 scrap in inventory so that whenever new upgrades become available, you can purchase them all immediately. Max upgrades all relate to ground combat, which is what the majority of the game consists of, so the more upgraded you are the less you'll die and the easier the game will feel. Despite needing to purchase all of the skills, there are some that are far more useful that others. For example, I found the shoulder charge to be entirely useless, and I never once used it in the game, whereas the skills to disarm enemies are absolutely critical to getting through some of the bigger and more difficult fights in the game. Make the disarming skills your first priority when they become available.

Note that, even though all the heads don't cost any scrap, you don't need to waste time going down the line and "buying" all of them for this achievement. Only the stuff that actually costs scrap needs to be purchased. Once you get towards the end of the story and finally purchase everything, this achievement will unlock.