I wanted to get this one as fast as posible, so here's what i did:
After gettin to level 2 with wilhelm i took the first skill, and i totally stopped killing the enemies, instead i only "pre-shot" my targets,by wich i mean criticals (pistol) to shatter the airmasks, and let my flying robots do the rest (had them flying around as much as i could). even when the bots were on cooldown, i only pre-shot enemies, jumping around to avoid there shots. I only got to level 6 and threw my few skill points into the "Auxiliary Tanks" skill, wich gives longer duration and faster cooldown rate.
In total it took me 1hrs and 28mins
happy hunting!
Thanks to Dan vasNormandy for pointing out that while using this method u can track your progress in the challenges menu: Action Hero should count upwards to 100 kills