The laser weapon is the second-highest (or possibly highest) level of power-ups you can collect during a level. Power-ups are crates with green parachutes, of course.
You will sometimes get the laser naturally by simply collecting lots of power-ups. But you can speed things along by using the main pilot's item summoning ability that unlocks for purchase after beating the third boss. Summon a power-up as soon as you start each level and continue summoning them whenever the meter charges up. This will get you the laser consistently, and then you'll be able to reach 200 kills in no time.