Assault Android Cactus

At first, this achievement had been very elusive. I believed it had something to do with score, but that's really not the case. You can look at the leaderboards and see scores higher than S+ ones with only an S rank.

The real condition is a lot more difficult:

You must FULL COMBO the level for the S+ rank.

Note: Being Downed ends your combo and your shot at the S+ rank.

The best way to know whether or not it's possible is that right next to your Battery meter is a S+ badge. If you have your combo meter break, then the S+ badge goes away.

So far after about 10 hours with the game, I have reached S+ on 3 levels. The first 2 levels as well as the first boss, Embryo. I've found personally that Starch and Coral worked well for me depending on the level setup. I would say focus on Level 1. You'll get the feel for it and figure out how to best tackle enemies and prioritize them.

One tip that I find worthwhile is to mix your focus at times. Some of the stronger enemies take too long to kill before your combo meter runs out. Therefore, what you'll want to do is hit the tougher enemies with a couple hits then switch to a quick kill weak enemy. After this, switch back to the stronger enemy.

Additionally, learn the enemy layout. Some enemies will appear at the same locations. You'll want to know a path in order to hit certain enemies and maintain the combo.

As an add-on to the above, I used Holly on level 1 as her seeker weapon made it easier to keep the chain going, especially when the flying enemies appear off screen.