I like your team. These are some of the changes I'd make.
Hippowdon:
I'd change him from an attacker to a wall, and give him moves like Stealth Rock, Slack Off, maybe Toxic, etc. The sand will hurt the defender over time and you can supplement the damage with rocks and toxic. I'm assuming this would be your starter.
I'd suggest more defensive EVS in Defense, Sp. Def, and HP.
Garchomp:
I'd go all out with this one. Garchomp has the ability to really put the pressure on with devastating moves, so I think you should just go for huge hits. Maybe sub out the leftovers for a life orb. I'd put 252 EVS in Attack and Speed, to make sure he can hit hard and strong. Even if Garchomp goes down early, you can be he'll be able to take at least one, of the opponents with him. Make sure to watch out with outrage on any team with a usable steel type.
Gliscor:
Make him more defensive-oriented. His main strength is defense, so don't put so many EVs into Attack. All something would need to knock him out is an Ice Beam, so maybe concentrate him more defensively. I like the moveset, though.
Mamoswine:
Good moveset overall, but if you take my earlier advice about using hippowdon as a starter, I'd switch out Stealth Rock for a good Rock-type attack like Stone Edge. I don't like using two stealth rock users on the same team- to me it just takes up space. You could also sub a life orb in for leftovers if you want more sweeping power.
Claydol:
With Claydol, I think you should go all the way with screens or just don't bother. I'd use one of the two sets below:
A. Screener: You could go full screens; Light Screen, Reflect, and Light Clay as your item (trust me, the clay is worth it). This leaves you with two other moves. Toxic and Earthquake are two good options, as earthquake can hit what toxic can't. The screens are very useful in a match, so I'd recommend this set.
B. Staller: Get rid of the screens and replace them with moves like Earthquake, Toxic, Protect, and Cosmic Power. This set takes advantage of those nice defensive stats. Set up with toxic and cosmic power and watch their clock tick down. Be careful of Frost Breath-that thing tears cosmic power claydol to bits.
Gastrodon:
This one's looking good!
Team:
There are some team type concerns: Water, Grass, and Ice are all super-effective against 4 of your guys, and are resisted by none of them. You have ice and fire for grass, and rock and fighting for ice, but you don't have any way of dealing with water other than poison damage. Add a grass or electric attack to one or two of your guys.
This is a pretty good team. I hope to have helped with this! Good luck!
