This section deals with effects that make your Pokémon require more hits to KO, or even become un-KOable in certain circumstances.
If a damage reduction effect applies before Weakness and Resistance, it resides on the attacker, and can be removed (if it was from an attack) by switching the attacker out. If the attacker damages multiple enemy Pokémon, the damage is reduced for each of them.
If a damage reduction effect applies after Weakness and Resistance, it resides on the defender, and can be removed (if it was from an attack) by switching the defender out. Swift-like attacks only ignore effects on the defender, not the attacker.
This reduces the damage that the attached Metal Pokémon takes by 10, and stacks.
One Metal Energy is enough to let a 100 HP Metal Pokémon (e.g. Gardevoir δ) survive two 50 damage Rayquaza ex δ Special Circuits, and reduces Dark Tyranitar's Spinning Tail's damage on that Pokémon to 10.
Some decks (e.g. Swampert ex, Pop, Quad Rayquaza δ, Dark Steelix) are built with the goal of eventually creating an unkillable tank with 3 or 4 Metal Energy attached to it.
Energy Removal 2, Pow! Hand Extension, or the Derail attack (found on Machamp and Exploud ex) can get rid of Metal Energy.
With a basic Lightning Energy attached, this reduces the damage that the attached non-ex Pokémon takes by 10, and stacks.
Usually, Holon Energy GL is played more for the Grass anti-Special Condition effect. The ex damage reduction is just a bonus that you can unlock by splashing Lightning Energy, so why not.
This reduces the damage that the attached Pokémon takes by 20.
Can be removed by Windstorm. Using a forced switch-in/out card will send the Buffer-ed Pokémon to the Bench, letting you attack the new undefended Active. However, if you're spreading damage to the Bench, the Buffer-ed Pokémon is still defended.
Double Rainbow Energy reduces damage done by the attached Pokémon by 10, and stacks.
"Why would I ever gimp myself by attaching more than one Double Rainbow Energy to my Pokémon?" Of course you wouldn't, but your Pow-using opponent will. If they Pow multiple DRE onto a Pidgeot for example, that Pidgeot becomes dead weight for attacking; Clutch will only do 20 damage or less, and Pidgeot's free retreat will come back to bite it, because it can't get rid of DREs by using them to pay for Retreat Cost. They can also Pow DRE onto a spread damage attacker to severely weaken its damage output.
(Note that Delta Reduction is an effect on the opponent's Pokémon that got hit by it, not Meganium.)
Healing is generally weaker than damage reduction, as it will not protect against an attack that does just enough to KO your Pokémon. If your (Metal) Pokémon has 80 HP and your opponent is attacking for 80 damage, putting Metal Energy on it would let it survive with 10 HP. Putting Heal Energy on it would not, as it's either at full HP, or already KOed.
These are one-time healing consumables.
Potion and Life Herb are only used in tank decks like Quad Rayquaza δ or Dark Steelix.
Oran Berry is a Castaway target. Chiro Special (the Basic ex deck that accelerates with Latias ex δ and redistributes the Energy with Mewtwo δ) uses it. The deck is full of 100 HP Pokémon-ex with Poké-Powers/Bodies, and Oran Berry lets them survive two Rayquaza ex δ Special Circuits.
Sitrus Berry is like an Oran Berry for Evolved, non-ex, non-Dark, non-Owner's Pokémon that heals more. While this sounds nice, Energy Root is often just better. Stage 2 decks don't have room for it, and Stage 1s have 70-80 HP, so if your opponent is attacking for 80 damage, Sitrus Berry wouldn't save you, but Energy Root would.
Combines the effects of the classic Full Heal Energy and Potion Energy.
Like Holon Energy GL, removing Special Conditions is the main draw. The 10 heal is just a nice bonus.
Healing Shower completely heals all non-ex Pokémon.
It's a good counter to ATM Rock, as devolving undamaged Pokémon will not KO them, and if they ATM Rock without getting 30 damage on Milotic first (KOing Feebas upon devolution), you can replay Milotic for a full board heal again.
Can be reused with Mr. Briney's Compassion, an already defensive card.
These function as slow healing cards. Given enough turns, you can replay the Pokémon that you scooped up along with any Energy attached to it, though any Rare Candy that was used to get it in play the first time around is not recoverable.
(Heals other Pokémon; note that Cursed Stone will make those with Powers fall behind:)
(Heals self; Baby Evolution omitted:)
(Calm Mind breaks even with Stantler's Push Away, and Ralts can't be Confused by Screechy Voice.)
Ice Barrier and Psychic Shield grant immunity from Pokémon-ex.
Aside from switching to attacking with a non-ex Pokémon, there are some other ways past them. Energy Removal 2 or Crystal Beach can keep them off the Energy that they need to use their attacks. Warp Point (or any other switch-out/in effect) can send them to the Bench, whereupon they'll lose their protection, and you can snipe/spread damage to them with an ex.
Safeguard is the most common ex immunity Poké-Body, but it can come in other guises, like Shedinja's Wonder Guard (immune to Evolved Pokémon too) or Shuckle's Extra-Tight (doesn't prevent effects, only damage).
There are ways to beat Safeguard without switching to a non-ex attacker. Space Center disables Safeguard on Basic Pokémon, Latias δ does the same for Evolved Pokémon (other than Dustox ex), and Gardevoir ex δ can Imprison any Safeguard Pokémon.
One trick pioneered by the Mewtric deck (which also works against Latios ex δ and Celebi ex) is to have Mew ex use Deoxys ex's Fastwave. This hits for Weakness against Latios ex δ, Celebi ex, and Wobbuffet, and 1HKOs them through any of their protection effects.
This peculiar pair of Pokémon is protected from damage of a particular parity. Certain decks are vulnerable to one or the other:
Odds: Rayler, Banette ex
Evens: Bombchamp, Kingdra ex δ, Arcanine
This grants Safeguard protection to any of your non-ex Pokémon for a turn.
In practice, it's incredibly fragile and countered by Windstorm and Warp Point, just like Buffer Piece.