There are multiple times where effects are evaluated in the attack procedure:
[*=1]After doing requirements like flipping a coin, we check for effects that alter or cancel damage calculations (for example "during your opponent's next turn...").
[*=1]Then we used to check for Confusion, but now we are doing that up front after announcing the attack.
[*=1]Then we do effects in the card text that are specific in timing and say things like "before doing damage..." or "you may discard an energy...".
[*=1]If we are dealing with damage counters we deal with that and stop, but if we are doing actual damage we now figure the base damage and then the damage additions from Trainers and such.
[*=1]If the damage is greater than 0 we process weakness and resistance. After that, we do the damage reductions from Trainers and such. Only after all of that is complete, we do attack effects like discarding energy and removing damage counters.
[*=1]After that we check for any effects that happen just before the KO check (in the order the players want them applied, active player first - like Rocky Helmet and Rough Skin, etc.).
[*=1]Now we check for KO all at the same time and then we go back to processing effects and apply any that happen after the KO (like Fainting Spell).
I know I have left steps out here, including effects that are evaluate between turns, but I have now listed 5 different times that effects are evaluated, depending upon the wording of the text or the cards played.
Back to the card at hand, Pachirisu has that wonderful word in it: "now". While ordinarily I would process this effect after damage, that single word "now" stares into my soul and glaringly says that it is an immediate effect that falls into either the "alters or cancels" pre-damage effect evaluation or the "specific timing" pre-damage effect evaluation.
My logic here is that "Now" is an adverb meaning "at the present time or moment" and that would seem to say that it counts "at the present time or moment" so the effect would be evaluated in one of those two pre-damage effect evaluations -- that is, for both the current attack AND until the end of your next turn.
So, net:net -- my confusion comes down to the word "now" in the text. It depends on what the meaning of "now" is. Does it mean what the dictionary says, or do Pokemon rules somehow trump it? If so, I would really, really like confirmation of when exactly this weakness change is evaluated and why exactly that is the appropriate time so that I can judge this kind of thing when/if I see them crop up at pre-releases and for tournaments like this year's Regionals with brand new cards where we have not been able to work all the questions out for each card.
Thanks for being there guys!