You don't really need to train your team for the Kalos Elite 4. It's extremely easy. So instead, I'll show you how to use your Pokemon to defeat them. But before you challenge the Elite 4, replace Shadow Ball with Shadow Sneak and replace Iron Head with Swords Dance on Aegislash.
For Malva, spam Surf with Greninja, easy.
For Siebold(I think that's his name, haven't played in a while), use Mega Venusaur and spam Energy Ball on his team until Gyarados comes out. Then switch to Heliolisk and use Thunderbolt.
Wikstrom will be easy. For Klefki, if Garchomp can't take it out with a single Earthquake, use Venusaur. But after Klefki, have Garchomp use Earthquake on Aegislash and Probopass. You won't need to worry about Kings Shield, as Earthquake doesn't have contact damage. And Flamethrower the Scizor.
For Drasna, however, she will be tricky. For her Dragalge, have Garchomp use Earthquake which will most likely OHKO. For Altaria, use Dragon Claw. But for Noivern, switch to Greninja and use Ice Beam. You should be able to outspeed and even if you move last, I'm pretty sure Noivern doesn't have any super effective moves against Gren(it might have Focus Blast, however). And for her last Pokemon(which I believe is Druddigon), try using Dragon Claw with Garchomp and then Ice Beam with Greninja.
Now for the easiest Champion ever. Notice how I never mentioned using Aegislash at all? Well now, it time for it to shine. Use 3 Swords Dances against Hawlucha, it can't do anything to you, then spam Shadow Sneak to win. Against Aurorus and Tyrantrum, use Sacred Sword.