This team is designed to counter the elite 4. Every pokemon in the elite 4 is countered in this team.
Scizor-Need to trade with a metal coat for this, this is perfect for elite 4 number 4
Magmortar/Infernape/Rapidash-For elite 4 number 1 Need to trade Magmar with Magmarizer from another game for Magmortar (he's the best option).
Empoleon/Gyarados-For elite 4 number 3, remember gyarados is a physical attacker so waterfall is better than surf and hydro pump. Empoleon has a weaker point to fire but doesn't mean it's not worth using, it's water/steel type make the fire type do x1 damage to it and not x0.5.
Abomasnow-For elite 4 number 2 and champion-He easily beats the ground type and beats Garchomp down to a blob with blizzard or wood hammer! Also good for milotic.
That is all you need really but here are some backup options if you wanna be safe:
Gallade/Bronzong-Nice backup psychic types (Bronzong preferably).
Scyther/Beautifly-Nice backup bug and flying types (Scyther preferably cause it's faster and stronger but has a small range of moves).
These pokemon should be level 50 or higher.
Items Reccomended: 10-15 Hyper Potions and 10 revives or 5 Revival Herbs.
Magmortar: Sunny Day, Solar Beam, Flamethrower, Psychic. Infernape: Sunny Day, Solar Beam, Flamethrower, Close Combat. Rapidash: Sunny Day, Solar Beam, Flamethrower, Any Move.
Scizor: X-Scissor, Iron Head, Sword Dance, Roost (Teach roost to scyther before evolving).
Empoleon: Ice Beam, Surf, Flash Cannon, Drill Peck. Gyarados: Waterfall, Ice Fang, Crunch, Aqua Tail.
Abomasnow: Wood Hammer, Ingrain, Blizzard, Ice Beam.
Gallade: Psycho Cut, Leaf Blade, Close Combat, Any Move. Bronzong: Iron Head, Gyro Ball, Faint Attack, Zen Headbutt.
Scyther: Aerial Ace, Roost, X-Scissor, Air Slash. Beautifly: Silver Wind, Bug Buzz, Morning Sun, Air Cutter.
Those are the best moves and Ice Beam is needed on Empoleon if you don;t use a back-up psychic type.
I tackled the platinum elite 4 with a level 49 scizor, a level 54 empoleon, a level 48 Giratina and a level 63 magmortar. Got as far as the champion and Garchomp keeps beating me I went in with 50 hyper potions and 15 max revives and faced the champion with 48 hyper potions and 10 max revives. I couldn't wait so I just bought a load of stuff and charged. Garchomp keeps Earthquaking my team and flamethrowering my scizor. My empoleon isn't fast enough to use Ice Beam and EQ keeps killing it in one hit. So all you really need is those 4 and perhaps gallade to beat him.