The easiest level in my opinion to get this level 4B, although it is possible to just get this naturally in other levels, also becomes easier the more you've upgraded your ship.
In level 4B, if you take the alternate boss route the boss (as shown in the video below) it is relatively easy. It only shoots asteroids at you and they are pretty easy to dodge especially if you stay at the far left of the screen.
Shooting the asteroids builds up your combo and providing you don't get hit you will get a pretty high score, almost guaranteeing you an S rank.
Also as a bonus, if you drag out out the boss fight as I show in the video by hanging around underneath and not shooting the boss too much (this is why I end up accidentally getting hit just to show the tactic as I wouldn't normally do this when trying to beat the level), in my opinion this is the best way to grind credits if you would like to upgrade your ship more.It is also easily got in the Boost Rush Mode Levels if you are able to pass them, I have also wrote guides for these.
Boost Rush Mode Level 1:
Boost Rush Mode Level 2:
Hope this helps and good luck!