First off, for Serperior, if you're trying to go for speed with it, don't give it the negative priority move in Dragon Tail. I'd go for the safe Toxic, so you can outspeed and get the poison on a lot. Second, Blastoise looks good, give it EVs in Defense so it can survive the first hit before you get the Iron Defense up. Magmortar, keep it the way it is, or, give it Choice Specs and then replace Solarbeam and Sunny Day with T-bolt and Focus Blast, gets the water coverage that way, and a way to get rid of bulky rock types. Fourth, Garchomp, I'd suggest replace Brick Break with Stone edge, it's just a better move for that really hard hitting set you're going for. Other than that, it's good. Bastiodon, I'd put it in the lead and give it Stealth Rocks instead of Metal sound, but that's just a personal preference. Solid choice. Finally, Electrode. Get rid of the Magnet Rise and give it an air balloon, and then give it Volt Switch. Electrode can outspeed almost anything in the metagame, so go with that. Get in, get the damage, get out. As for items.....
Serperior- either Razor Claw to increase the chance of Leaf Blade's crit even more, or Life Orb
Blastoise- leftovers, make it even more of a wall
Magmortar- either Choice Specs if you get rid of sunny day and Solarbeam, Power Herb if you want to keep Solar but not Sunny Day, and Salac Berry for your set
Garchomp- Choice Band or Life Orb. no description necessary. \
Bastiodon- Leftovers, same as blastoise
Electrode- Air Baloon if you change Magnet Rise, or Focus Sash
Side note, you might want to switch Blastoise or Bastiodon, one or the other, to a Specially defensive Pokemon, you're a little lacking in that category. Hope it helped!