In a format where Mew VMAX and Gardevoir ex are some of the most dominant decks in Japan, Koraidon ex might have some steep competition
If you have a lot of relevant permanents such that their Detention Spheres always have targets but yours might rarely have non Detention Sphere targets, you might need Banishing Light so that you can remove their Detention Spheres with a card that would otherwise be dead, having no other target than Detention Sphere
Undercity is an awesome dungeon designed to be better for multiplayer than the original cycle of dungeons from Adventures in the Forgotten Realms
From that moment onwards it is as easy as to keep spamming your attackers and spreading damage counters
Card sets: base and jungle
Im pretty undecided on how good this card will be, but with the format trending toward one where incidental treasure builds up like unwanted house guests, giving an opponent two Lotus Petals is probably a lot less of a problem than it might have been five years ago