When Leotheras hits 15%, he pauses to talk a bit and his demon form splits off to be offtanked. If you manage to kill Leotheras before the Shadow of Leotheras spawns, the shadow will remain after his normal form dies. You can then run down the hall and jump in the water and the shadow will eventually reset without properly locking you to the encounter. Wait outside the instance for ~30 minutes for a soft reset and you can repeat the process. If you have a fresh lockout, you can probably skip straight to him and zone out+reset to the same end.
Drops 250g, T5 gloves and a few other epics which change every kill.
Kill #1 (this is actually the second kill for this lockout though)
Still unsaved
Kill #2 (incredibly unlucky dodge/parry string, didn't kill him in time to go again)