Tabletop
He will move his echo as a free action, switch places as a bonus action, and then look to attack the Plague Fiend.
- Attack #1 - Hit AC 25 for 18 slashing and 3 fire damage. The damage is not halved.
- Attack #2 - Hits AC 19 for 13 slashing and 3 fire damage. The damage is halved.
- Attack #3 - Hits AC 27, which I will guess is a soft critical, for 36 slashing, 6 fire damage (if just a regular hit, that's 22 slashing, 1 fire); damage is not halved.
That's a total of: 71 damage