I don't know if you are accepting character ideas, but here is my pitch.
Boss: Mount Fortmore-HP: 10,000, Damage: 15, Def: 20
Skills: Battle cry-spooks some team members for a short time, Wait is 10 turns
Summon-calls for aid during the battle, wait is 15 turns
Attacks: Vacuum-sucks in someone from the team and for one turn the trapped teammate will receive damage.
Spit-Spits out debris in a pattern that can be blocked and countered (If a perfect block is achieved, that rock will fly back to the mountain)
Defeat strategy: Each of the 4 heads has 2500 HP, a trapped teammate can be released before damage is dealt if damage is dealt to that head.