Just beat the game for the first time! And wow,was easy to beat. HisGhetsiswas the easiest thing to set up on with Lilligant (with a few Antidotes), and then Petal Dance outspeeds and OHKOs his whole team. Mine was only level 51, too...Cofagrigus
And I'd been slightly worried because RNGing Zekrom meant I only had 4 of my real team with me.
Why did you only have 4 of your team? When you catch zekrom/reshiram it asks if you want to put it in your party and send another poke to the PC. So, if your team had 5, and you used a synchronizer, just drop the synchro.