Coilfang Ambusher can be found in Serpentshrine Cavern.
These mobs spawn as adds in the The Lurker Below encounter. When The Lurker Below dives, he spawns a total of 9 adds: 6 ranged Coilfang Ambushers (2 on each platform), and 3 melee Coilfang Guardians. All these adds can be Crowd controled.
Have about one group on each Ambusher pack. They should be tanked by your DPS warriors/feral druids or even Rogues and High-HP mages if you don't have enough people.
Attacks and abilities
Coilfang Ambushers - 26k Health. They have nasty burst damage and can Multishot for about 2.5k on leather.
They are currently very buggy and can sometimes despawn for no reason. Sometimes a healer will get aggro on them from the Lurker platform and they will start evading until the healer jumps over onto the Ambusher's platform. Sometimes, if the Lurker emerges while an Ambusher is evading someone, the Lurker will despawn, so be careful.