Rate My Team Zero tries to chop down the Battle Tree

From what I can see here, there are a lot of flaws but it is a great start. Let's just go over the team one Pokémon at a time and see what your team struggles with.

Umbreon is pretty OK, I much prefer Leftovers for extra sustain but Rocky Helmet aint bad. I feel that if you want to run Rocky Helmet on Umbreon then go with Bold nature with Max Defense EV's so it can take physical moves better and retaliate better with Foul Play.

Buzzwole needs a revamp. Having both Hammer Arm and Drain Punch on the same set isn't great and you are just asking to get walled by Fairy types with Outrage. Put Poison Jab and Ice Punch instead of Outrage and Hammer Arm and max Attack and Speed and you should be alright. I'd also recommend putting Earthquake on instead of Leech Life to handle certain matchups better like Heatran and Toxapex.

Aerodactyl isn't the best Mega in OU but it's a pretty decent wallbreaker, personally I'd go with either Mawile or Tyranitar for your mega as they can go a lot longer than Aerodactyl but they are slower so it's your choice. If you do go with 'Dactyl then I'd have Aerial Ace, Stone Edge and Ice Fang instead of Iron Head, Rock Slide and Aqua Tail. EV's are fine.

Nidoqueen isn't the best thing to bring into OU as a rocker, you'd be better off using Ferrothorn, Landorus-T or Heatran for rocks but if you wanna use Nidoqueen that's the set I'd use.

Xurkitree needs a revamp as well. Zurkitree isn't suited for defensive purpose at all with a mediocre base 71 Defense and Special Defense stat. What Xurkitree does well however is being a special nuke. How do we exploit that? Z- Hypnosis. Z- Hypnosis raises Xurkitree's speed by 1 stage (a 1.5x boost) and you can put something to sleep which sets up for part 2 of Xurkitree's nuking strat, Tail Glow. If your opponent has no way to act out of sleep, they have to switch, meaning you get a free Tail Glow to raise your Special Attack stat by a whopping 3 stages. Then you can proceed to sweep everything with Thunderbolt and Energy Ball. Xurkitree is usually best brought in after something faints on your side because Xurkitree really cannot take a hit well at all and Earthquake really messes it up.

Serperior is good but if you don't want to use it it can easily be replaced with something that will give Xurkitree a crutch when dealing with Ground types like Landorus and Garchomp. Tapu Bulu. Bulu is one of Xurkitree's best partners because of Grassy Terrain, it reduces the power of Earthquake and Earth Power by half so Xurkitree can take a hit, powers up Xurkitree's Energy Ball and it gives Xurkitree a little bit of sustain with the passive recovery each turn. Bulu does well with Choice Band/Scarf and even Assault Vest.

Spoiler: Banded Bulu Set

Tapu Bulu @ Choice Band
Ability: Grassy Surge
EVs: 252 Atk / 4 Def / 252 Spe
Adamant Nature
- Wood Hammer
- Horn Leech
- Superpower
- Zen Headbutt

or

Spoiler: Assault Vest Bulu

Tapu Bulu @ Assault Vest
Ability: Grassy Surge
EVs: 248 HP / 28 Atk / 36 Def / 52 SpD / 144 Spe
Impish Nature
- Horn Leech
- Wood Hammer
- Superpower
- Nature's Madness

Really up to you to decide what you want to do with Serperior or your choice of Bulu set.

Now with those changes your team is pretty well rounded but it still struggles against a few things.

Heatran
By far the biggest weakness to this team, with Bulu on the field Earthquake and Earth Power won't damage it too much and you really can't touch it without a Water type. Banded Bulu's Superpower will KO all variants of Heatran but offensive Heatran will outspeed Bulu so you have to catch it on a switch in.

Rain
Mega-Swampert and Pelipper are really good at offensive pressure and they aren't afraid to expose weaknesses. Without Grassy Terrain, Xurkitree cannot kill Swampert as in the rain, Swampert will outspeed Xurkitree, even with the Z-Hypnosis boost and without Grassy Terrain, Xurkitree cannot take an Earthquake so it's dead. However, Bulu can take on Swampert ans it can take one Ice Punch but Offensive Pelipper will take it down with Hurricane. Bulu however can outspeed it and KO it with either Horn Leech or Wood Hammer so as long as Bulu is alive you'll be ok.

Koko-Lucha
Tapu Koko and Hawlucha have disgusting synergy and it's not easy to take out Hawlucha when that Unburden boost is active. Hawlucha can and will outspeed and KO your entire team with Acrobatics and Drain Punch/Hi-Jump Kick. If Hawlucha gets up a Swords Dance and lives, it's game over. You can avoid this by pressuring Xurkitree's use early, causing Hawlucha to not be ready for an impending sweep from Xurkitree. Putting Hawlucha in a bad situation is great for the team and KOing Hawlucha is even better, Koko isn't really a problem because Bulu exists.

Hazards
Hazard stacking is going to be your team's worst nightmare. Ferrothorn can pressure you with layers of Spikes and if that team has a Toxapex, OOF. Spikes stacked with Toxic poison is HELL and this team has no Defoggers or Rapid Spinners to stop the setup. You can avoid this by swapping Umbreon for Espeon so you can bounce the Toxic Spikes and Spikes back or giving up a moveslot on Aerodactyl for Defog. Not to mention...

Ferrothorn
You. Cannot. Kill. Ferrothorn. There is no way you can handle Ferrothorn efficiently. Once again, keeping Bulu alive is VERY crucial as at this point he is the backbone of this team as Bulu has a chance to OHKO Ferrothorn without Chople Berry with Superpower. Consider having a fire type like Heatran as a rocks setter as opposed to Nidoqueen.

That's all I can think of at the moment. The team is good but it definitely has it's flaws. Hopefully this helped and if you do have further questions feel free to "@" me.