Verified answer

Xerneas is tournament banned, and thus should never be used. Throw it out.

Breloom is a very powerful Grass/Fighting type, and if you can import one from Generation V that has Technician, it can sweep entire teams. Make sure it knows Bullet Seed, Spore and Mach Punch.

Dragonite or Salamance are both tournament legal, have great coverage, and have access to STAB Outrage. Consider using them.

Blissey, using Heal Bell, Softboiled and Seismic Toss, can heal your team and whittle down your opponents. If you have an Eviolite and don't care about using Leftovers, a Chansey could do the job better.

venasuar? not really to sure what other grass pokemon would be a good fit for your team but other pokemon would be good is crookadile dark and ground with ability moxie.