Decklist from LittleRiver.
This deck uses a variety of Pokémon to attack the opponent. For raw power, Exeggutor δ and Rayquaza ex δ have attacks that conditionally do 70 damage for 2 Energy, but that's not all this deck has going for it. This deck has two spread damage attackers (the same Exeggutor δ and Rayquaza ex δ), three Pokémon that can copy opposing attacks (the Mews), two ways of shutting down Poké-Powers (Cessation Crystal and Jirachi ex), and one free retreater/ex counter (Latios ex δ). It has five different types of attackers, so it can frequently exploit Weakness for double damage.
Play Castaway and Lanette's Net Search early to gather the resources you need to attack. Both of them are net +2s in card advantage, and get the stuff you need early-game. Castaway can search for a Supporter, which is going to be Lanette's Net Search more often than not if you don't already have it.
Lanette's Net Search gets three different types of Basics from your deck, including Holon's Castform, which acts as an Energy card. Pick Pokémon that hurt your opponent the most, whether it be something that hits for Weakness, Jirachi ex to shut down their Powers, Latios ex δ against ex's, or Stantler to be annoying. At worst, it prepares 3 Bench-warmers for Exeggutor δ's Delta Circle.
Sometimes Jirachi ex or Latios ex δ will virtually win you the game by themselves. Other times, you'll have to work for your meal. Use Rayquaza ex δ or Stantler to soften them up (or Exeggutor δ if they really have a lot of HP) while you prepare another attacker on the Bench. This could be Exeggutor δ for its raw power, a Mew to hit for Weakness, or another Rayquaza ex δ which will be able to attack for cheap after one of your Pokémon gets KOed.
Since Rayquaza ex δ has more than 100 HP, Holon Mentor can't be used. This deck uses a Castaway toolbox instead to set up in the early-game, and to chain Supporters in the mid- to late-game.
Two common early-game situations that Castaway + Lanette's Net Search finds all the pieces for are:
If you need to stop opposing Powers: search for Psychic Energy and Jirachi ex, or Cessation Crystal and a non-ex (Stantler is a good choice).
If you need to retreat your starting Pokémon: search for Balloon Berry, or, if you started with Latias δ, then Latios ex δ.
This deck doesn't use Evolution-searching cards, instead opting to draw into Exeggutor δ with its 6 draw Supporters.
This deck uses a 3-2 split of Mary's Request/Holon Adventurer. Holon Adventurer can be used to discard dead late-game cards, like unused Pokémon or Lanette's Net Search. You can also discard any δ for it to be a net +2.
This deck has a fairly high basic Energy count for Castaway, which also allows Multi Energy to provide all colors most of the time.
The one Holon's Castform nonbos with Crystal Beach, but this is pretty minor and can be played around.
Mew ex and Pokémon Retriever act as a virtual 2nd/3rd copy of any Pokémon you have. If you need to maintain Jirachi ex's Shield Beam while your opponent is trying to muscle through it with damage, you can have Mew ex in the Active Spot copying a benched Jirachi ex's Shield Beam, and they'll have to get through two 90 HP Pokémon to free themselves. (Note that you can't get both Mew ex and Jirachi ex off a single Lanette though, since they share types.) Alternatively, if you have Pokémon Retriever and Latios ex δ, you can have Jirachi ex Shield Beam directly, and when it gets KOed, send out Latios, Retrieve the KOed Jirachi to hand, attach Psychic/Multi Energy to it, retreat to Jirachi, and continue Shield Beaming.
The non-δs in this deck are Mew ex, Jirachi ex, Stantler, and Castform. When counting the number of δs you have for Delta Circle or Delta Draw, skip any Psychic- or Colorless-types.
Warp Point: An extra switch/minor disruption card.
Scott: This can get two Stadiums if you're going to fight that war, or two Supporters to ensure you'll have one after an opposing Stantler's Push Away, or two Holon Mentors against Banette ex to power up a copied Shadow Chant.
Another Jirachi ex: If you don't want to risk the lone copy being Prized. Remember you still have Cessation Crystal even if Jirachi ex is Prized, though that is more fragile.
Water Energy: For Latios ex δ's attacks.
This deck has many ways to attack the opponent. Brute force, spread damage, Power lock, Push Away, Ice Barrier, Dual Aura — they're all there.
All Pokémon in this deck (other than Exeggutor δ) are Basics, so it's pretty easy to get set up and attacking, and Lanette's Net Search provides maximum value every time you play it.
This deck is generally favored against Stage 2 decks. Jirachi ex attacks for {P} if they have a Stage 2 out, and shuts down Jirachi and Pidgeot, which they often play. Rayquaza ex δ can 2HKO snipe Pidgeot with Special Circuit. If their Weakness permits, a Mew can revenge kill their Stage 2 efficiently.
This deck plays a decent number of ex's, which can turn into 2-Prize liabilities if your opponent goes after them with spread damage attacks. Desert Ruins damages Rayquaza ex δ and Latios ex δ over time. An opposing Rayquaza ex δ can 2HKO snipe Jirachi ex, Mew ex, and Latios ex δ. (If Latios gets Ice Barrier off, it's immune to Rayquaza, but the Rayquaza user can remove the immunity by Pow-ing Latios to the Bench.) Be careful when putting these down against opponents that can prey on them; try not to be tempted by extra damage on Exeggutor δ's Delta Circle or Latios ex δ's free retreat if they're not necessary.
This deck's maximum damage output in one turn is 70 (sans copying), meaning it's weak against DPS check decks like Rayquaza δ or Dark Steelix. Additionally, some Stage 1 decks like Scrambled Eggs or Banette ex have Stage 1s with more than 70 HP and no convenient Weakness to exploit; those are tough too.