This Breaker randomly spawned while I was attempting to clear the Lobert Draw Ridge Horde on Highway 97. Breakers are the toughest Freakers in Days Gone. They can be real bullet sponges so your best option is stealth. However, you will need to unlock the Executioner skill (final skill in the melee tree) in order to steath kill them.
This Horde adds up to 400 Freakers, 100 short from the Old Sawmill Horde.
Also if anyone else had the same issue where you need to trigger the remainder of the horde to respawn in the tunnel, please let me know in the comments.