You will find Marcellis and Alessia on the bridge to the northeast of Somborn Village around ( -555, 15, -79 ) /waypoint -555.29, 15, -79. Marcellis attacks after a few seconds. He's a Heroic ^^^ but I managed to kite him each time. Bring a group for best results*. Around 20% health he will despawn.
Once they despawn you will be timed on each of the following steps.