If you chose Chespin, it'll be easy to beat Tyrunt. Froakie can have a bit more difficulty with Tyrunt, but it has an easier time with Amaura. Keep Fennekin away from the battle.
If you managed to catch a Honedge, or you can snag a Hawlucha, you've good as won. Honedge walls Amaura and can deal with Tyrunt fairly easily (watch out for Bite). Hawlucha wipes the floor with Tyrunt, and while it is weak to Amaura, it can still do massive damage.
If you allow trades, grab a Luvdisc if you can and trade for a Steelix in Cyllage. Neither Amaura nor Tyrunt can do much against it, and it'll slaughter them both.
Crippling them with status works well. Grabbing a Skitty to Sing them to sleep can completely demolish them. If you picked Bulbasaur at Sycamore's Lab, Leech Seed/Poison Poison Powder can make short work of them.
The EXP Share is absolutely broken in Gen VI, so you can just grind if all else fails.
Hope I helped!