This can seem like quite a daunting task at first but save for a few levels, it really isn't. Firstly, if you have the two Bonus Levels then you can do either of these instead of one of the regular levels. This allows you to skip up to two levels that you may be having trouble with. Secondly, there are two ways of getting an A+ on any given level. Third and possibly most importantly, if you attempt a section of a level and you don't like how it is going, you can die to restart the section with no penalty. This allows you to essentially start from a checkpoint and maximize the run for each section rather than needing a whole perfect run of the level. (NOTE: Do not hit the "Restart" button from the menu because this restarts the entire level!)
Probably the simplest and quickest way to do this is to play through the game once and return after you've collected the Zack mask from getting a high score on Chapter 13. This will give you an idea of the enemy layouts before tackling this. The Zack mask increases the amount of time you have before your combo expires. From there, try to clear each section with one long perfect Combo. This will almost certainly get you the A+. There are quite a few videos on Youtube to help you do this.
The second method can give you more points per kill rather than requiring a combo. This doesn't net as many points as a combo that involves every enemy on the level but it can be used in spots where the level isn't conducive to that. If you knock an enemy over first, you get credit for knocking them over and then a good chunk of points for executing them. Do this to maximize your points per kill.
While both of these can be used individually, you can also use them together to really gain huge scores. If you can kill every enemy in this method and combo them all together, the score will be through the roof. Also, this method of killing takes a little bit longer and it can be used to stretch the amount of time your combo lasts so that other enemies are in a better position for you to continue your combo.
Getting this does require familiarity and expertise on your part. There are some tricks and strategies that can help you but if you can't execute then no amount of tricks will help you get there.