To get S+ rank, you have to kill all the enemies with the combo up, meaning you have to kill the first enemy, chaining enemy after enemy, until the last one.
IMPORTANT: Search in Youtube "Assault Android Cactus S+" and you can watch a REALLY useful guide. I won't link it here, but is pretty much the first result you get in the Youtube search.
Switching between main weapon and special weapon, gives you some frames of invincibility. Androids jump to switch, at that moment, they are pretty much invincible. Use that in your favor.
Bosses can be tricky (but they are predictable, unlike the rest of the levels) and the last zone is pretty hard to be honest, but If you practice a lot, this will come eventually. I am going to comment briefly Zone 1, 2, 3 and 4 and which androids to use.
Zone 1 - Zone 4: For all of the regular levels, use Holly. Her main weapon is one of the best in the game in terms of keeping the combo up, is not as powerful as others, but as long as you defeat the hardest enemies with your special (cannon ball), you can really do work here.
Zone 1 - Zone 4 BOSSES: My best advice here is to use Cactus or Strach. Cactus special (flamethrower) is just amazing, and Strach is very balanced.
Zone 5 is really hard as I said. These are some tips I wrote as I was doing it. Sorry I did not do the same for the first 4 zones.
5-1: Cactus: Her special is way too powerful and you should spam it like hell. Be very careful when bombs and bomb containers come out. Wherever the first comes out, the rest will follow, so just shoot in that direction until you clear that wave.
5-2: Peanut: I love her main weapon and the drill is very usefull the scape some situations. At the end of the level, use a lot of blue power ups to paralize the enemies. I stayed at the bottom of the level, using the machine gun dron you can use in your favor.
5-3: Holly: I hate this level so much. The floor was not a big problem, but I kept losing my streak. Just practice and practice, it took me a couple of hours to S+ this level.
5-4: Strach: The first part can be hard, because the combo easily drops. Just kill red drones until you get a power up (blue or red). The rest is pretty easy until Liquorice. Be careful with her shots and keep moving so you can avoid her special. SPAM A LOT of Strach special vs Liquorice. A lot of the rockets will hit her making the fight really easy once you get the hang of it.
5-5: Strach: To be honest, it was really easy for me. The hardest parts probably are the bosses in between, but with red power ups you can melt them easily. Use LT to avoid blue shots from Medulla between bosses. VR section has always been easy for me, but I can tell you that being accurate with Strach main, and spamming her special, will make this part very easy. If you feel nervous when you get to this part, just pause, breathe, maybe watch some Youtube footage of the VR part, and go for it calmly. I did it with like 15-20% of my battery remaining, so time is not a problem if you are calmed.
You can use comments to ask for help and I will gladly help you. This can be a frustating ride, remember to be patient, and If you get stuck in a level, swtich or get some fresh air. Good luck!