Does Mewtwo's "Pressure" Ability block all of Tapu Koko's "Flying Flip" Attack's damage, even to the bench?
Pressure says, "your opponent's Active Pokemon's attacks do 20 less damage." Since it does not specifically designate Active or Benched Pokemon it reduces damage by -20 to each Pokemon that would be damaged by the attack.
Source: TPCi Rules Team (2018-07-26)

Relevant cards

Note: All applicable cards may not be listed below.
SMP_SM30
SMP_SM30
SMP_SM30A
SMP_SM30A
SMP_SM31
SMP_SM31
SMP_SM77
SMP_SM77
XY10_78