Paladin of the Snow Queen
The first time it happened was in Ultra Space. I'm presuming it absorbed some sort of Necrozma energy from the wormhole and now just powers up when it has enough again.Naganadel has literally zero training or battle experience, so...
If it beats Lucario, then that's easily the single weakest Lucario ever seen in the anime
Pikachu has access to three Electric Z-Moves:
Gigavolt Havoc - any electric move
Catastropika - Volt Tackle (already showcased by Pikarla)
10,000,000 Volt Thunderbolt - requires Thunderbolt and an arbitrary hat and is activated whenever the plot needs it and is never explained how or why it happens - Ash's Electric-Z magically turns into Pikachu-Z for no reason then reverts
Naganadel has little to no onscreen experience, but it learned Thunderbolt on its own so it's clearly been training in the magical world of offscreen land.
Ash's Pikachu doesn't have access to Catastropika unless Quick Attack becomes Volt Tackle again.