I find that the best way to beat later levels is to make a tank with just a single large bubble and stuff it with spread weapons and an area burst cannon, and then just circle around the perimeter of each room while aiming at the middle to sweep all the enemies while avoiding a lot of their bullets. 100 health isn't an issue when the enemies have a hard time hitting you and you can recover after every room.
 "Bubble Tanks Arenas"){kind=icon}