I will give you that at low lvls marksman skills have mad annoying cooldowns. However as you lvl Marksman skills their cooldowns lower making it a lot better. Lvl 5 gatling rush has a 9 second cooldown and the grenades has a 2 second cooldown, and gatling really helps with respawns. those two alone I can comfortably boss with, so I don't want you to think they absolutely fail at bossing.
However I do find F6 better as I totally can just solo one side.