Resistance is severe in RS-PK compared to later eras of the game. It's -30, compared to BW, XY, and SM Blocks' -20. Damage numbers in RS-PK barely reach the heights of those blocks, and when they do, it's not without considerable setup.
If you rely on only one attacking type, you might get walled by Resistance, though some colors have it harder than others.
Power through it. If you hit hard enough, you don't care that your attack is resisted if it still ends up 1 or 2HKOing the opposing Pokémon.
Have secondary attackers of different types. Bonus points if they hit the Weakness of Pokémon that resist your primary attacking type.
Have attacks that ignore Resistance (Brick Smash, Swift, anything that hits the Bench), put damage counters on opposing Pokémon, or Confuse them.
Have damage-over-time effects: Poison, Burn, Cursed Stone, Desert Ruins.
Have a way to win by decking your opponent out first (e.g. Sniff Out + Rocket's Admin, Rocket's Wobbuffet + Pokémon Retriever, Beating Wings Pidgeot + Celebi ex loops).
Hit for Weakness by: Fire, Colorless
Other options: Crystal Shard, Beautifly
Hit for Weakness by: Lightning, Colorless, Darkness
Other options: Crystal Shard, Magnetic Storm
Holon Energy FF is more limited than it seems. If your attacker is an ex, it doesn't get the Resistance-ignoring effect. If your attacker isn't an ex, but is an Evolved Pokémon with a 3-Energy attack, it's likely using Double Rainbow Energy. Adding a basic Fighting Energy and Holon Energy FF for that would be overpaying.
Hit for Weakness by: Fighting, Colorless
Other options: Crystal Shard, Magnetic Storm
Note that Latias δ, ☆, and ex resist both Psychic and Fighting, so adding a Fighting-type secondary attacker might not be enough.
Hit for Weakness by: Fire, Colorless
Mono Water-attacking decks are rare. Latios ex δ or Mew ☆ are only a handful of attackers in a larger toolbox, and something like Gorebyss has a bigger problem in the form of its Weakness to Rayquaza ex δ.
Hit for Weakness by: Colorless, Fire
Lightning Resistance is not too common. All Lightning resists are Evolved Pokémon (Vibrava, Flygon, Steelix ex), so Stantler can deal with them.
Hit for Weakness by: Fighting
The two mono Metal-attacking decks are Scizor ex and Dark Steelix. Scizor ex against a Metal-resist turns into a wet noodle fight, as the Scizor will have Metal Energy and Steel Wing to reduce damage. If its HP drops to 60 or less, Danger Perception gives it a damage boost to start plowing through Resistance. Unboosted Cross-Cut is capable of 2HKOing Manectric ex, Jolteon ex, and Magneton.
Dark Steelix has Darkness Energy to power through Resistance.
Hit for Weakness by: Colorless
Fire is only resisted by the Salamence line, which is weak to Colorless. Salamence is not all that commonly played, though.
Hit for Weakness by: Darkness (only Gengar ex and Dusclops ex)
Sableye has no Weakness.
Nothing resists Darkness in the format. Man, Dark Tyranitar and Dragtrode have it easy...