Purugly would be a Physical Sweeper. I don't think you should change it at all, although you should Ev train it in 252 Atk., 252 Speed, and 4 Hp.
Quagsire: Replace Dive with Waterfall, much more useful. Have it hold maybe a Mystic Water or Soft Sand and Ev train it in 252 Def, 252 Sp Def, and 4 Hp.
Garchomp: One of my favorite Gen 4 Pokemon. Get rid of Sandstorm, since this isn't a sandstorm team and replace it with Earthquake. Replace Fire Fang for Brick Break, or keep it if you want. Definetely take Dragon Claw and replace Aerial Ace for Rock Slide or Stone Edge. Ev train it in 252 Atk, 252 Speed, and 4 Hp.
I don't think you need Breloom since you have Garchomp as a Special Sweeper, but keep it if you want. If you want something different, then I would suggest Alakazam as a Special Sweeper. Get it the Magic Guard ability, and give it the Life Orb (deal extra damage but don't get damaged by the Life Orb)! Teach it Psychic, Shadow Ball, Focus Blast, and Charge Beam. Ev train it in 252 Special Attack, 252 Speed, and 4 Hp with a Modest nature. If you want to keep Breloom, the moveset is great, and Ev train it in 252 Attack, 252 Speed, and 4 Hp.
Altaria: I don't think you need two dragons, so I think you should replace it with Shuckle. Yes, that's right, Shuckle. Teach it Stealth Rock (Gen 4 Tm), Toxic, Mud Slap (accuracy lowering hax), and Wrap, mostly just to keep the foe in and let Toxic take it's toll. Ev train it in 252 Def, 252 Sp Def, and 4 Hp. Give it an Impish nature and have it hold the Leftovers.
Metagross: Keep either Meteor Mash or Bullet Punch. Both are STAB. Bullet Punch has priority, and Meteor Mash has higher power, lower accuracy, and a chance to raise Attack. Both are great moves. The one that you don't keep, replace with Ice Punch. Ev train it in 252 Attack, 252 Def, and 4 Hp.
I wish you many wins and I hope this helps!