For Decidueye, I think you should run Long Reach over Overgrow if possible. Generally speaking, having contact moves not make contact is more useful than powering up Leaf Blade when Decidueye is low. Also, I would invest the 4 HP EVs into defense instead. It only raises your HP by 1 point anyways, so I think the point in defense is more useful because it makes it more likely to tank physical hits, versus HP not really helping with defense or special defense.
For Charizard, as sm 4evr mentioned, should run Timid over Jolly, because you are running a special set. I agree with sm 4evr that you should run Mega Charizard Y and Solar Beam as your fourth move. It charges in one turn under sun, which compliments Mega Charizard Y's ability very nicely. It also provides coverage against Water types. Just be careful of Primal Kyogre. You don't want Kyogre switching in and forcing you to waste a turn charging Solar Beam.
For Swampert, don't run Assault Vest and Stealth Rocks on the same pokemon. Alternatively, Leftovers could be a good replacement. Also if you're going to invest max EVs in a defensive stat, I would definitely suggest making 252 HP your highest priority, even if it isn't your highest defensive stat.
For Tyranitar, you should probably use Chopple Berry as your item, because it allows you to take fighting attacks you wouldn't normally survive for 1 turn, allowing you to set up Dragon Dance. For the other 2 moves, you might want to use Pursuit (it can heavily punish pokemon trying to switch out.) and Superpower (for coverage).
For the other 2 pokemon, there are a few options:
Yveltal @ Leftovers
Ability: Dark Aura
Bold Nature
252 HP/252 Def/4 SpD
IVs: 0 Atk
-Foul Play
-Toxic
-Defog
-Roost
This set is meant to Defog/Stall. Pretty self explanatory.
And I noticed you don't have an electric type, so I stole this set from smogon.
Tapu Koko @ Electrium Z
Ability: Electric Surge
EVs: 252 Spe/248 SpA/8 Def
-U-Turn
-Thunder
-Taunt
-Nature's Madness
