well, just start it like normal pkm game- but remember u have to pick and train 6 pkm in the same time to deal with the gym leader. Gyarados is OP but in double battle, still can be KO before u can hit anything after dragon dance. trust me, a double battle with gym leader is hurt. bcz we all have lots of exp with single battle through 6 gen, but double battle in this game is so annoying.
btw, u shouldn't care about EV in the early time. just pick pkm with good nature and "acceptable" IV. bcz u can do mostly nothing about that. ex: when your op send out magnezone, mostly u HAVE TO send infernape. u can't say:"oh no, i need my infernape got atk. and speed EV, not sp.atk" when u got ditto and save the old man in day-care---> breed it. then u can train for EV later. i mean, later in game, IV is the most important thing to pick. u can change nature of pkm. and u got ability capsule. so it's not problem. p/s: i got blaziken adamant nature with speed boost and 31 IV atk from the start (def/ sp.def/ speed IV are 10/7/18--->but it doesn't matter right
and that blaziken got:
-bulk up (with sp.boost, it's like quiver dance in physical side )
-brave bird
-high jump kick
-flare blitz
THAT IS OP+ SUICIDE. trust me, much better than chimchar. mostly fighting gym leader my blaziken WILL DIE. But bcz he kill himself :v :v and of course he take out 3 to 4 pkm with him :v