Alien Swarm: Reactive Drop Cheats on PC

Enable developer console and press [`] (tilde)
After that, make sure to input the following commands:

asw_ignore_need_two_player_requirement 1
asw_skill_grenades_hornet_count_base [number]
asw_skill_health_base [number]

Replace number with a desired amount.
It is recommended setting health to 50000 and hornet count to 5000.

This way you will be able to fly through the missions and ultimately get xp.

Note: If the cheats do not work, enter the console sv_cheats 1
code before entering these codes.