Every time they go around you loose health, and they upgrade an elite tier and get stronger (Icons in how to play), when a boss goes through, its an insta kill. So you shoudn't let them through, but some weaker enemies have to go around a few times to get one shot capability.
Bosses spawn starting from round 10, its a bit challenging, but a good strategy of magic orbs can beat the first ones easily.
Thanks for the feedback though.