Your Side Deck, consisting of up to 15 cards, is where you can include additional tech choices to swap into your Main Deck between games in a match
When it comes to exactly what Rookies you want to build for your deck, there are tons of options, but a deck recommended by YouTuber TrueChampionSteven specifically caught our eye
Jolteon ex LC - Flash Spear 60+ You may discard up to 2 Basic Energy from your Benched Pokmon
Aim to pull staples and meta-relevant Pokemon so that you can build decks for Ranked Battles

Advanced Search Sets Random Syntax Anchovy & Banana Pizza Annihilating Glare Assassin's Ink Assassin's Strike Avatar of Woe Bake into a Pie Bala Ged Scorpion Banewhip Punisher Betrayal of Flesh Big Game Hunter Bitter Downfall Bitter Triumph Black Hole Blight Grenade Blind Zealot Blood Curdle Bloodchief's Thirst Bone Shards Bone Splinters Bounty Hunter Brainspoil Breathe Your Last Cavalier of Night Certain Death Chain Assassination Claim the Precious Come Back Wrong Common Black Removal (playtest) Consign to the Pit Contract Killing Covert Cutpurse Cruel Cut Crumbling Ashes Cut Down Dalek Drone Damn Damning Caress Daring Demolition Dauthi Cutthroat Deadly Alliance Deadly Derision Deadly Embrace Deadly Plot Deadly Precision Deadly Visit Death's Caress Defeat Defenestrate Demon-Possessed Witch Desperate Plea Devour in Shadow Dirge Bat Disembowel Disemvowel Disruptive Stormbrood Dokuchi Silencer Downwind Ambusher Drag to the Underworld Dragon's Prey Duh Easy Prey Eater of Hope Elektra, Daughter of the Hand Eliminate Elspeth's Nightmare Embrace Oblivion Eviscerate Exterminate

Feel free to try it out if the playstyle interests you, but don't pick it just because it sounds cool and then wish you'd gone with the massive dragon deck instead