Seeing as you aren't running Sandstorm, I would suggest not using Excadrill. While you have a decent chance of running up against a Sand team, the rise of Rain and Sun have made it so that Sand will not be on every other team. You'll want to replace it with something better equiped to deal with Conkeldurr. Since you have two Pokemon week to Ice already (so no Gliscor) I would go with Cofagrigus.
Eelektross: This guy is not needed when using Thundurus. Since Thundurus is the better of the two, replace Eelektross. I would add Ferrothorn for some good resistances and also some Stealth Rock help.
Hydreigon: Give it a Choice Scarf, give it a 252 Sp Attack, 252 Speed, 4 whatever (not Attack unless using U-Turn) EV spread. Replace Surf for Dark Pulse and Flamethrower for Fire Blast. Dragon Pulse you can replace with Draco Meteor since you are running a choice item. make it your lead. Also, you can use u-Turn over Focus Blast.
Thundurus: Give it a Life Orb and 252 Sp Attack, 252 Speed, 4 whatever (not Attack) EV spread, a Timid nature, and Thunderbolt, nasty Plot, Focus Blast, and Dark Pulse.
Drapion: Use this
Drapion (M) @ Black Sludge
Trait: Battle Armor
EVs: 156 HP / 252 Atk / 100 Def
Adamant Nature (+Atk, -SAtk)
- Toxic Spikes
- Ice Fang/Poison Jab
Terrakion: Give it a 252 attack, 252 speed, 4 whatever (not Sp Attack) EV spread with a Jolly nature. Replace Swords dance for Earthquake.