Pachirisu's "Trick Sticker" attack says, "the Defending Pokemon's Weakness is now {L} until the end of your next turn". Does that mean the weakness is applied "now" before the damage is done for this attack?
Nice try, but no. Like all attacks that don't explicitly say otherwise, the effects of attacks are not applied until after damage is done.
Source: TPCi Rules Team (2015-05-07)

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XY4_25