Cannon Master Willey

Scarlet Bastion
Work through the Hall of Lights to Malor, then continue to Willey's cannon room and Galford's archive before the final chamber. Dathrohan's transformation begins the Balnazzar portion of that last fight, not the end of the branch. Blacksmithing Plans along the Bastion route can summon Crimson Hammersmith; treat the plans as an optional pull when encountered, not a required stop after Balnazzar.
Cannon Master Willey
Elite HumanoidWilley tests whether the party can keep the boss anchored while a cannon operator removes the Crimson Riflemen.
Abilities

Knock Away
Inflicts normal damage plus 30 to an enemy and knocks it back.
Tank with solid scenery behind you so Knock Away does not throw you out of the cannon room's established position.

Pummel
Pummels an enemy for 45 damage and interrupts the spell being cast for 5 seconds.
Keep the healer out of Willey's melee reach so Pummel cannot lock out a needed heal.

Shoot
Shoots at an enemy, inflicting Physical damage.
Willey also shoots ranged players; the healer must cover the cannon operator as well as the tank.

Summon Crimson Rifleman
Use the room's cannons on the summoned Crimson Riflemen. Keep Scarlet Cannonballs ready instead of allowing each firing line to accumulate.
Tank- Anchor Willey with your back against solid scenery and leave the cannon route clear. Do not drag the boss onto the cannon operator while recovering from Knock Away.
Healer- Stay outside Pummel reach while keeping the cannon operator in healing range. Riflemen and Willey's Shoot can damage that player even when the tank is stable.
Damage- Put the assigned cannon operator on riflemen duty with Scarlet Cannonballs ready. Other attackers stay on Willey unless a reinforcement threatens the operator or healer.