Can you please tell me what Pokemon you used to beat the Elite Four and..

I can't remember mine exactly but the best thing to do is train your Pokemon the same and have different kinds so don't just have a load of fire or anything like that spread it out so like have well I chose a ghost type, a rock type, a steel type, a fire type, a water type and a ice type it worked for me seeing as you can use different pokemon which are more effective on their Pokemon!

Me?
I used a:
Groudon lv. 56
Blaziken lv 54
Regi ice lv. 54
Rayquzza lv 71 (this was traded so get a Dragon Pokemon!).

I would say if you use that team then you will beat them!
(if you can't get the exact Pokemon then get the same types!)
Groudon-must have!
Fighting
Fire
Water/ice
Dragon
Maybe dark-my Rayquzza knows crunch!

I've used Blaziken, Peliper, Manetric, Gardevoir, Wailord and Groudon.
Blaziken lv 50
Peliper lv 48
Manetric lv 48
Gardevoir lv 48
Wailord lv 40
Groudon lv 46

I only used one Pokemon it was my lvl 79 Swampert that knew surf dive earthquake and ice beam. I had a pretty easy time.

Mine were -
Swampert lv 70:
Surf, mud shot, earthquake, endeavour.
( still training these)

Kyogre lv 59:
Rest, waterfall, ice beam, hydro pump.

Lairon lv 40:
Roar, take down, iron tail, strength.

Manectric lv 40:
Odor sleuth, roar, bite, thunder.

Loudred lv 40:
earthquake, super sonic, stomp, screech.

Wailord lv 40:
Rest, whirlpool, dive, water pulse.