When dealing with damage modification effects, it helps to view the attack as a chain:
- Base damage
- Effects on Attacking Pokemon
- Weakness/Resistance
- Effects on Defending Pokemon
The damage starts at the top (base damage), runs through any effects on the Attacking Pokemon, then it travels over to the Defending Pokemon, where Weakness/Resistance gets applied, and then any other effects on the Defending Pokemon. Whether a damage modification effect is applied before or after Weakness and Resistance is the key to telling which Pokemon the effect lies on.
Extra Smoke
Any damage done to your Stage 2 Pokemon-ex by your opponent's attacks is reduced by 10 (before applying Weakness and Resistance).
Extra Smoke is applied before Weakness and Resistance. That indicates the effect lies on the Attacking Pokemon -- which is your opponent in this case, since Extra Smoke reduces his attacks. Swift only ignores effects on the Defending Pokemon. This is why Swift still gets reduced.
Here's another example from Lucario:
Close Combat (80)
During your opponent's next turn, any damage done to Lucario by attacks is increased by 30 (after applying Weakness and Resistance).
During your opponent's next turn, he'll be attacking Lucario. Since this effect is applied after Weakness and Resistance, and Lucario is the one taking the extra damage, the effect lies on Lucario. Which means Swift ignores Close Combat as well
Let's look at Deoxys (Normal) HP:
Crystal Laser (20)
During your next turn, Deoxys's attacks do 40 more damage to the Defending Pokemon (before applying Weakness and Resistance).
You can probably guess that the effect is on Deoxys, but this helps to reinforce the overall concept. Deoxys is creating the damage in this case, and the modification is before Weakness/Resistance.
Here's an interesting one from Deoxys (Defense) HP. This was the errata Delta Reduction. This was the original wording on the card:
Delta Reduction (30)
During your opponent's next turn, any damage done to Deoxys by attacks is reduced by 30 (before applying Weakness and Resistance).
The errata was to make it more clear that the effect lied on the Attacking Pokemon:
During your opponent's next turn, any damage done by attacks from the Defending Pokémon is reduced by 30 (before applying Weakness and Resistance).
But you can see, the rule applies even with the "bad" wording. Before Weakness/Resistance always means the effect is on the Attacking Pokemon, which is your opponent's Pokemon in this case.
Effects on attack damage are among the more complex effect interactions in Pokemon, and even a lot of judges don't quite get it. But it's deceptively simple:
- Before Weakness/Resistance => Effect on Attacking Pokemon
- After Weakness/Resistance => Effect on Defending Pokemon
Remembering those two things goes a long way to properly interpreting an attack.