Can I use Umbreon-GX's "Dark Call GX" attack to discard 2 Double Colorless Energy?
Yes, you can select 1 Energy of each Double Colorless Energy to discard both of them. This applies for discarding your own Energy for an attack and for retreating as well. So if you used Hoopa-EX's "Hyperspace Fury" attack or paid its Retreat Cost of 2, discarding 2 Double Colorless Energy would be legal options.
Source: TPCi Rules Team (2017-01-26), TPCi Rules Team (2017-06-27)
Say my opponent used Turtonator GX's "Shell Trap" attack last turn, then on my turn I attack it with Tapu Bulu GX's "Tapu Wilderness GX" attack. Does "Shell Trap" place its damage counters before or after the healing part of "Tapu Wilderness GX"?
Tapu Wilderness GX heals Tapu Bulu GX as part of its attack before the damage counters are placed for Shell Trap's effect. So in the end Tapu Bulu GX will have 8 damage counters on it.
Source: TPCi Rules Team (2017-06-08)
Say my opponent used Turtonator GX's "Shell Trap" attack last turn, then on my turn I attack it with Shaymin EX's "Sky Return" attack. What happens to the 8 damage counters effect from "Shell Trap"?
Sky Return puts Shaymin EX into the hand as part of its attack before the damage counters are placed for Shell Trap's effect.
Source: TPCi Rules Team (2017-06-08)
If Vileplume's "Allergy Panic" Ability is in effect, does Rayquaza's "Shred" attack consider a Defending Pokemon's weakness as x2 or x4?
Shred isn't affected by "any effects on the Defending Pokemon", but the term "any effects" does not include effects that alter Weakness or Resistance. So in this case the Defending Pokemon's Weakness would be x4 due to Allergy Panic.
Source: TPCi Rules Team (2013-02-28), TPCi Rules Changes Announcement (2017-05-03), TPCi Rules Changes Announcement (2017-05-18)
What happens if I have less than three spaces on my bench when using Politoed's "Roll Call" attack? Or what if I don't have any room on my bench at all?
You can use the attack no matter how many Pokemon are on your Bench, but you can only get as many Pokemon as you have spaces on your Bench.
Source: Guardians Rising FAQ (2017-04-22), TPCi Rules Team (2017-05-04)
What happens if I use Politoed's "Roll Call" attack and cannot find a Poliwag, Poliwhirl and/or Poliwrath in my deck?
When performing an attack you do as much of it as you can. And since the contents of the deck are unknown you can also voluntarily fail to find one or more of the Pokemon you are searching for.
Source: Guardians Rising FAQ (2017-04-22), TPCi Rules Team (2017-05-04)
What happens to Gothitelle's "Tractorbeam" attack if the opponent has no benched Pokemon?
If you can't switch a new Pokemon, the attack doesn't do any damage.
Source: Guardians Rising FAQ (2017-04-22), TPCi Rules Team (2017-05-04)
On the turn after using Dragonair's "Dragon's Wish" attack, when I attach any number of Energy cards from my hand do I have to attach them all at once, or can I attach them whenever I want during my turn?
You do not have to attach them all at once. You can attach them from your hand whenever you like during the duration of your turn following Dragon's Wish.
Source: Sun & Moon FAQ (2017-02-02), TPCi Rules Team (2017-02-02)
Can I have Clefairy's "Metronome" copy a GX attack, and if so does it use up my "one GX attack per game"?
Yes you can, and yes it does. Also if you've already used your GX attack for the game, then you cannot choose a GX attack with Metronome.
Source: TPCi Rules Team (2017-01-05), Sun & Moon FAQ (2017-02-02)