When Hector enters the boss room in Mortvia Aqueduct, he will be presented with a long bridge he must cross with the promise of a new Innocent Devil's room seen in the distance. While stepping on the middle part of the bridge, however, this will suddenly crumble, sending him down to a circular platform at the bottom of the room which is surrounded by water.
The Skeleton Diver will then make his dramatic appearance, riding on a gigantic skeletal fish and jumping high off of it to land over Hector, trying to impale him with his trident. When the battle properly starts, the player will immediately notice that it is not the best idea just trying to fight him head on, as his skillful combos will always give him the upper hand in a one-on-one fight. The hunch on his back, on the other hand, is his weak spot and if Hector manages to position himself behind him, the Diver will get staggered with each hit until he cowardly jumps back to the safety provided by the fish.
The fish won't just stare there while the fight goes on and it will in fact aid his master, becoming a major nuisance throughout the battle by performing two main attacks:
- It will come out of the water, perch itself on the shore and unleash a powerful laser beam which will cover most of the battlefield. The player should try to get to either of its sides (left side preferred) and attack the fish itself. All of this while also fending off the Diver's relentless attacks.
- The fish, this time ridden by the Diver, will emerge from the water and land on the ground. It will then start hopping on the surface, making its way to the other side while attempting to stomp on Hector.
While riding the fish, the Skeleton Diver will submerge underwater; after a moment, the fish will leap high from the water while the Diver on its top performs one of two attacks, either trying to land on Hector with his trident or throwing a set of fire-infused tridents vertically, which will impale on the floor and remain there for a brief period of time.
His last attack is riding the fish once more but this time circling around the arena while hurling tridents at Hector. This is a good opportunity to attack him. Hector must perform single or double quick steps to avoid the tridents and, at the same time, trying to position himself on the fish's path to attack the Diver while he passes in front of him. The sword- and hand axe-types' second combo (two normal attacks followed by a final attack) is recommended for this purpose, as it sends the weapon flying forward in a boomerang-like fashion which deals continuous hits.
When the fish emerges from the water and starts hopping on the ground, the Skeleton Diver (riding on its top) will open his stealing window. The player must carefully position in a spot on either side of the fish's path, anticipating its movements and leaving enough space to maneuver, lest they end up cornered; then jump and attempt to steal, taking care of not being knocked back while doing so.