Whenever you copy an attack from a Pokemon, you treat any reference to that Pokemon as a reference to the attacker.
So when you use Omniscient to copy Sky Spear, it says "place all energy attached to Arceus in the Lost Zone". So you would perform that condition for your attacking Arceus, not the one on the Bench.
Also, make note that you aren't putting the energy in the Discard Pile, but the Lost Zone. That's important.