Ugh...my "Ditto trick" didn't work.
As I tried to experiment it on that Suicune from Route 25...and it seems that on the status screen, the stats of Ditto is still the stats of Ditto itself, not the stats mimicking the target poke (Suicune).
Maybe the only way is to use seismic toss/night shade pokes.
For Reshiram, I might use Level 87 poke with night shade/seismic toss.
Because a 30/31 HP IV Reshiram has 175.
Or maybe use a Poke with speed that's one point below the target...Hmmm...
My favorite catcher is a Gallade. Teach it False Swipe, Mean Look, Hypnosis, and Sword Dance (to increase False Swipe damage versus resistant Pokemon). Though you'll want to bring some X-Accuracy and X-Defend items to boost Hypnosis pitiful accuracy, and Gallade's terrible Defense stat.
Also for Victini, if you're going for a flawless Victini, a level 29 or 58 Night Shade/Seismic Toss user is perfect. If it survives one level 58 Night Shade/Seismic Toss, or two level 29 Night Shades/Seismic Tosses (without using Endure), then it's got a flawless or near-flawless HP stat.
Victini and Reshiram/Zekrom, it's very helpful to use a 1-under Speed Pokemon as well. Will save you a lot of time in the long run.
I also found that Bisharp is an adequate catcher for anything that's not a Fighting or Fire type.