Anyway, passive healing via Aqua Ring, forcefields via Protect, Mega Launcher boosted, Rain boosted Water Pulses that leave the 8 tails confused, versatility, Mirror Coat to just give back all that damage twofold, 8 Tails doesn't hold out too long, especially because 8 Tails is much simpler than Killer B himself.
Keep in mind that all of this base Blastoise can do. It's the closer AP that makes it fairer.