Vertical shooting game =/= mobs should come in from the top.
When i play as an archer in green wood, i wait for those mobs which appear from the bottom of the screen to move up before hitting them. Con of this is that i will be swarmed by bullets before i could kill them which is bad as archers don't have any bullet clearing skills. To counter all this weird spawning points, i kinda memorise where the mob spawn at so i won't get a surprise attack from them.
As for getting swarmed by mobs, i always concentrate my attacks at the point where the mobs crowd together and hell fire it. Even if i couldn't clear them all, at least i clear most of them. For bowmen, phoenix could solve the problem easily for you.
All hail OS~