Meganium

Decklist from KG.

Overview

This deck uses Meganium ex to power up a variety of attackers. Aiding it in consistency are Pidgeot, which requires no introduction, as well as Holon Mentor and Chimecho δ. Holon Mentor can get any Basic attacker or engine piece, or Holon's Castform to act as Energy, and Chimecho δ retrieves basic Energy from the discard pile, essentially negating the discard cost on Holon Supporters.

Ideal Gameplan

Lead with Holon's Castform and use Delta Draw to set up. Your first goal should be to get Meganium δ or Pidgeot into play. Once you get either one down, it can search for the pieces required to build the other.

While you're building up, use cheap attacks to put pressure on your opponent. Meganium δ's Delta Reduction does 40 damage for 2 Energy, and helps you survive longer by reducing the damage the opponent's Pokémon does. Latios ex δ can wall opposing ex's with Ice Barrier. Roselia can use Flick Poison to mess with the opponent, switching their setup Pokémon out of the Active Spot or racking up residual damage to get opposing Pokémon into 1HKO range for your other attackers.

As you're using these cheap attacks, build up a secondary attacker. You'll want something that can do huge damage, be it a Latios ☆ that 1HKOs Stage 2s, or a Pokémon that hits the opponent for Weakness. If all else fails you can use something as mundane as a second Meganium δ or Pidgeot, or a Meganium ex that you can afford to invest 5 Energy into. After your opponent deals with your current Active, send in the backup attacker for the revenge kill. 

If you're trading KOs with your opponent, you should be favored. Evolutionary Call keeps the pressure on, and Mr. Briney's Compassion can be used as a defensive card if they're unable to 1HKO you. Against ex's, Latios ex δ walls them and forces them to spend resources to get around it. If you run a non-ex into your opponent's ex, they KO you, then you KO them back, you're ahead 2 Prizes to their 1.

Unique Points

Meganium δ's Delta Reduction puts an effect on the opponent's Pokémon. This means that if they want to get rid of it, they have to switch or evolve their Pokémon; switching out your Meganium doesn't remove it. The damage reduction applies to ALL damage that they do, including damage to your Bench, so if they use a Spinning Tail that would hit your entire Bench for 20, Delta Reduction reduces that to 0!

This deck plays Mr. Briney's Compassion, but note that it cannot be used on ex's. You can use it to get non-ex Pokémon out of Pow traps. Latios ex δ can't be trapped if its Poké-Body is active, so that leaves Meganium ex — try not to play it if you know your opponent can Pow it Active and you can't turn the tables on them by using it as an attacker. Try to keep a Chikorita/Bayleef on your Bench at all times so you can replay Meganium immediately after Briney-ing it.

When you have Chimecho δ in play and a basic Energy in hand, your Holon Supporters effectively become free to play. In other decks, you might have to discard a δ Pokémon to Holon Adventurer for maximum value, but here you can discard a Grass Energy instead and get it back with Chimecho δ for the same amount of card advantage. If you need to get a basic Energy into your hand, you can play Holon's Castform to bounce one that's attached to your Pokémon.

Latios ex δ does not reduce Latios ☆'s Retreat Cost to 0.

Tweaks

A second Holon's Magnemite/Holon's Voltorb: As a search target off Evolutionary Call. If you search for the first Holon's Magnemite off Holon Mentor, then your deck has no 1-Energy Holon's Pokémon left. Suppose a few turns later you've got an attacker with no Energy attached in play, and you play Meganium δ, looking for Energy to attach to it. Holon's Castform won't work because it'll bounce itself, but if you had a second 1-Energy Holon's Pokémon in your deck, you can search for that + Castform and attach them in that order. However, this point is moot if either 1-Energy Holon's Pokémon ends up Prized, which has a 19% chance of happening.

Latias ex δ, Rayquaza ex δ, Mew ☆, Steelix ex: Alternative attackers. Latias ex δ does huge damage for a modest Energy cost, and can accelerate Energy onto itself (or Latios ex δ, but not Latios ☆). Rayquaza ex δ can target the Bench, but its 2 Retreat Cost and the lack of Balloon Berry in this deck makes it a bad starter. Mew ☆ is strong against lock decks, as it can copy resource recovery/discarding attacks from opposing Pokémon. Steelix ex can snipe opposing Pidgeots.

Different Grass/δ Rainbow Energy split: Meganium ex, Chimecho δ, and Latios ☆ only work with Grass Energy, but δ Rainbow Energy is handy if your opponent has a Crystal Beach that's color screwing your Holon's Castforms. You can adjust the ratio or total number of Energy based on your attackers. 

Strengths

This deck can run a wide attacker suite due to Holon's Castform and δ Rainbow Energy fixing colors. You can tech different attackers to your liking.

This deck is very good at out-resourcing the opponent. Evolutionary Call easily finds you the pieces you need to build up another Stage 2, or Holon's Castform for Energy. Holon's Castform itself is pseudo-card advantage, as it compresses 3 Energy into 2 cards (itself + any other Energy card or Holon's Magnemite). Holon Farmer and Chimecho can get back Energy or Pokémon from the discard pile. Compared to Pokémon Retriever, Holon Farmer has the advantage of being a Holon Supporter, so if you need to use Farmer a second time, Holon Transceiver or VS Seeker can act as the second copy.

Meganium δ's Delta Reduction is very good against spread/snipe attacks. When two of the top decks in the format are Bombtar and Rayler, this is a valuable trait to have. In addition, it 1HKOs Stantler due to Weakness.

Weaknesses

This deck plays many Pokémon with Powers, making it soft to Cessation Crystal and/or Cursed Stone. You need 4 Windstorm against those cards, and I've even included Ancient Technical Machine [Ice] as a 5th counter.

This deck's damage output is underwhelming if you're not hitting for Weakness or blasting them with a 150-damage Shining Star. Your highest unconditional damage, if we exclude Meganium ex's Power Poison due to its steep Energy cost, is 60 for 3 Energy (Meganium δ's Mega Impact or Latios ex δ's Hydro Splash). Use Latias ex δ if you'd like more consistent high-power hits.