Evolved Creeper Boss
It is similar to a standard Evolved Creeper but has two additional heads, similar to a Wither Boss.
The Boss attacks by spitting TNT blocks at the player; While not as dangerous as placed TNT, one must be careful should it destroy one of the Ceeper Eggs found around the room. Should one be broken, a Evolved Creeper will give chase.
The boss, however, cannot be defeated directly and thus its shots must be deflected in a similar manner against a Ghast for which it will get damaged by its own deflected shots, losing its heads in the process thus making it weaker.
Should it lose both its heads, it adopts some sort of armor to boost its defense.