The Transient

The Transient is an Act 3 elite that gains huge Strength every turn but has a Fading counter - it vanishes on its own after a few turns. This wants you to actually kill it before it fades. It has a big HP pool and hits like a truck by turn 3-4, so you need front-loaded burst: block the early hits and dump as much damage as you can in the first couple of turns. Any class with a strong opening (Strength Ironclad, front-loaded Silent, Wrath Watcher) can do it. If it fades before it dies you get nothing, so prioritise damage over caution.