This isn't exactly intuitive. By using
, you can change your ship between blue and red. Your ship gets damaged by the opposite color, but absorbs bullets of the same color. To easily get to 100, work up to the first boss, then switch to blue and just keep absorbing bullets. The blue laser can't hurt you when you're blue, either.
To get the Tesla Shock achievement, you need enough absorbed bullets, and then press
. You have to absorb bullets to charge the Tesla.
To be clear, this does not have to be done one GAME, just don't quit out of the game. Otherwise the total accumulates across plays.