A pretty cool boss, basically a giant with a ghast head. The ghast can shoot fireballs that do bigger explosions than normal, and they both have some resistance. Also, since the ghast may hurt itself by fireballing players that are close to the giant, I added health to the ghast to help it survive better.
I recommend killing the ghast part first.
Also, you will need a command block because of the length. This boss should not disappear either.
Edit: I added totems of undying to the giant