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Mamoswine: I chose Mamoswine as my lead because he forces many common Gen. 4 leads to switch out, given me the oppurtunity to set of Stealth Rocks. With Stealth Rocks out of the way Mamoswine becomes a liable sweeper as well, easily OHKO Dragonite, Flygon, Salamence with 4x effective STAB Ice Shard. Earthquake is another STAB move that goes hand in hand with Mamoswines 130 Base Attack power, mortally wounded Metagross and OHKO Heatrans even with Shuca Berry. Stone Edge is to take care of Gyrados switch-ins that would other wise set-up on Mamoswine.

Snorlax: Along with Snorlax being a great special tank he can also be a physical attacker. With Snorlax's bulk and 110 base attack Snorlax makes for a great counter switch-in. Body Slam is a great STAB move that deals hefty damge and a chance to spread paralysis around giving Snorlax the chance to outspeed someone. Crunch deals with Ghost types that would try to switch into Snorlax's Body Slam then set-up, such as special sweeper Mismagius, Earthquake OHKOs Heatran and Magnezone while dealing respectiable damage to Metagross and Jirachi.
Selfdestruct doesn't really need an explanation seeing as it's a finishing move that is also a STAB OHKO almost every pokemon in Gen. 4

Dusknoir: Is my Physical counter, switching into most Physical attack pokemon and walling them, forcing them to switch. He deals with Scizor and can survive a Bullet Punch from a +6 att Scizor. He also cripples the opponents physical sweepers who don't switch out by using Will-O-Wisp. Recovering HP using Pain Split Dusknoir can keep comign back and doign damage as well as walling. Fire Punch deals with Scizor and other steel types, while Earthquake is Dusknoirs strongest attack and deals decent damage to most pokemon that doesn't have resistance to it. The only downside is that he has a wimpy 45 base speed and even though he is a tank only has 45 base HP giving him a total of 294 hp with 252 evs in HP.

Vaporeon: Vaporeon walls Infernape who would other wise come in a sweep my whole team with Close Combat and Overheat. Vaporeon also brings my low hp pokemon back into the game with Wish as well as doing good damage with STAB surf and crippiling Dragons, flyings, and Grass switch ins with Ice Beam.

Heatran: Heatran can come in against Choice Band Flygon who would switch in and U-Turn back out into another sweeper as well as switching into to those predicted fire moves and being boosted up by Flash Fire. Fire Blast is Heatrans big STAB move dealing great damage to any pokemon then trys to switch in, it also has a chance to burn pokemon. Fire Blast also deals 20-30% with a Flash Fire boost to Blissey. Earth Power is to deal with other Fires and steels and is also the counter move to another Heatran that trys to use Fire Blast on my Celebi.
And Explosion is there to OHKO some random pokemon when Heatran is at low hp.

Celebi: Celebi makes a great wall against Water, ground, and other grass types. Celebi can survive a Starmies Ice beam then Grass Knot it for a OHKO. Hidden Power Ice deals with dragon switch ins and other grass types. Celebi is also my Breloom counter who would also sweep my team given a chance.

Thanks for the advise 3nodding

Well, facing Machamp/Hariyama/Infernape, doesn't scare me to much. I'm not stupid enough to keep Mamoswine in against them. Against Machamp/Hariyama I switch into Dusknoir and Will-O-Wisp whatever they do. And if its Infernape I switch out into Vaporeon.

Dusknoir against Tyranitar ain't to big a deal. Crunch won't OHKO Dusknoir so I can try my luck for another Will-O-Wisp or switch out into Vaporeon. Against Gyrados I can switch into Celebi/Vaporeon, although Vaporeon won't do much unless I give it HP electric. And when posed against Gengar I can switch into its Shadow Ball with Snorlax and Crunch it, unless Focus Blast manages to One hit me.