I made a compact timer that is easily customised.

i made a giant zombie riding an enderdragon

You can make a new type of boss with it =/ Like for example you can use resource packs and player heads to make a mob looks like something else,then you summon a bunch of inverted slime riding it with a mob riding the inverted slimes,once you killed that mob the inverted slimes will fall through the ground along with the mob beneath the ground in which you can detect using pressureplates or trip wires (inverted slimes have a negative size PS make them invisible and invincible or else they won't do no good).
You can also stack a commandblock minecart on top of those inverted slimes and have rails beneath the floor to make cool custom attack