Just hit over 96% on my first go today grabbing me the Laser Assassin achievement. The only thing i did differently this time was to use overdrives during the level (i hardly ever use them normally) and on the last set of boss fights, instead of trying to kill the boss as fast as possible i made sure i took out all of it's "items".
For example, on the boss that spins the square things things at you, i took out every one of it's arms before it died. Like wise on the very last boss, i sat there tapping fire as fast as possible aiming at the little "ships" that it shoots out. I'm pretty sure each of these phases are timed so whether you're hitting the boss rings or not, it will take you into the final phase. Once you're in the final phase, the rings are gone and the boss constantly spits out rockets, i just took the boss out as soon as possible.
The video on google is a good watch, take note of which enemies he taps fire on and doesn't use combo's, you have to combine these 2 methods to hit over 95%.
After getting over 95% on all areas, it also unlocked boss rush mode which is needed for an achievement.