This achievement requires you to purchase all the upgrades for your robotic arm. As stated in the walkthrough on here: buying all the upgrades will cost 6070 Moon Crystal Ore If you are going for the achievement for $100 million in the bank or all the difficulty achievements needed you will have no issue getting the ore needed.
Some of the upgrades are available by default but to unlock the 3 other modes you need to perform 3 gigolo missions (2 with Natalia and 1 with Koharu).
The 1st gigolo mission is with Natalia (Mondo Girls 1) and unlocks the drill (which is needed to find all Scarlett Locations btw).
The 2nd gigolo mission will be with Koharu (Mondo Girls 2) and unlocks the freeze weapon
The 3rd gigolo mission you need to win prior to being able to buying all the upgrades will be Natalia again and will unlock the charge gun.
Just to throw it out there: It doesn't take too much money to buy the gifts needed to pass the 3 gigolo missions so you won't really be too much in the hole for the $100 million if you decide to get the subweapons early. The extra sub-weapon variety also help you out on very hard if you know how to use them and are much cheaper than buying the costume powerups. That being said, I didn't struggle too much to get past very hard with just the base gun and the drill (using the drill only for scarlett blood re-ups).