Ok, Doomlord judges.
Another idea.
Mass Shockwave
Behemoths loads 3 seconds and then casts multiple waves of shockwaves forward(5 waves for example). Every other wave has 4 shockwaves and other 5 shockwaves. Each shockwave deals 4,500/6,000/7,500/9,000 damage. The boss is vulnerable while the event is on and can die.
And another idea.
Ice Storm
Boss starts to channel for 15 seconds. Selects a random player's unit every 3-6 second within 800 range and damages it every 0,25 second 300/400/500/600 damage. If there's no player's unit nearby heals himself instead every 0,25 second by 300/400/500/600 health.