Well, the Dark won't help you now.
With the new rules, Dark Energy won't add damage to Magikarp's attack.
Even with the old rules, all you'd be doing is KO'ing your own 'Karp. After all, there are two inbetween turn phases after you attach the Dark and if your opponent does any damage to you, even if you flip heads on the Focus Band, the Darkness would place a damage counter before your next turn starts. Since it's not an attack, the Focus Band, if it's still there, won't save you.
With only 4 Pokemon in here, the one KO will probably be enough for you to lose the game. Even if you manage to get more 'Karp's out, they won't last more than a turn or two each.
I hope you were kidding with this deck.