I don't do the battle and trade thing, haven't done it since Gen one. I believe that in trading Pokemon unavailable in the game it removes the challenge and uniqueness of that game. Nevertheless I do like to provide information where I can so with as few spoilers as possible here goes.
1) Shauna will meet you in the post game and trade you an egg with her starter in it, this means if you want to have a full water, grass, fire wheel pick the Kanto starter Serena's partner matches.
2) a party of six combee is your best bet for completing the pokedex as a great number of Pokemon in the wild are the second and third stage. First get a strong combee and have the other five tagging along, it's best that they are all male and all possess the gather honey ability. Get into weak battles and check your Pokemon every two or three fights, usually anywhere from two to five of the combee will be holding honey, this allows you to stock pile honey to attract hordes of Pokemon and capture low stage Pokemon like Mareep on route twelve.
3)with the new super training breeding is a recommended use of your time, be sure to catch the new bird Pokemon fletchling as it evolutionary stage as talon flame often has flame body which cuts the egg cycles in half.
4) use the master ball of the version Pokemon, all the other special Pokemon are in caves and dusk balls work fine.
5) Lapras has a unique surf sprite and can learn waterfall, surf and strength, farfetched can learn cut and fly, with these two in your party your other Pokemon can concentrate on battling the baddies.
6) the number of thuder stones is limited and mareep gets a mega evolution, fot this reason think hard which Pokemon you want to evolve with a thunder stone. I recommend eevee.
7)scouring the routes leads to rewards in the form of awesome Pokemon. There's a ton in this game despite the list of new Pokemon being low.