I played it while on tribulation mode. I don’t know if this makes a difference. This has never happened to me before.
Essentially when he was near half HP while in his wizard form.. I reflect a projectile back at him.. then all the sudden he turns into his dragon form and starts flopping around. I just start swinging wildly because I have no idea what’s going on. The chest appears and I loot it. Then I notice that the Storm Wizard is back in his normal form shooting his projectiles at me. I just waited for the timer to end and got my achievement. One thing I noticed was that the 300 baubles or whatever that normally appear didn’t appear with the chest.
