The easiest way to do this is against the first boss. The level is fairly simple, and you can see the whole surface so you don't get stray enemies flying into you from behind.
The boss spends most of it's time with it's shield up, and you can't harm it. Try and get up close to the boss when the shield comes down, and blast it for all it's worth while staying far enough away so that the enemies that spawn around it don't get you. It'll explode every so often and wipe out the nearby surrounding enemies, so make sure to back up when you see it's health indicator up top by the timer get near empty.
Rinse and repeat, but try to anticipate the different enemy patterns ( you should know how the different enemies behave by now ) and be out of their way so you can keep shooting at the boss and not need the smart bomb.
Eventually the boss starts chasing you around and you really have to work hard to keep out of it's way. Try going in wide circles away from it, while shooting back at it, and hopefully if you don't crash into anything, you'll nickel and dime it to death and get the achievement. Good luck!