In the normal games, literally all you have to do is wait until evening, Thunder Wave, False Swipe to 1HP, and start chucking Dusk Balls. Legendaries usually go down in under 20 by that point.
As opposed to constantly having to aim at a moving target, with nothing better than an ultra ball, losing hundreds of balls in the process. Hell, out of those 40 ultra balls I threw at Zapdos, I think maybe 15 or 20 actually landed, and out of those, only maybe 10 or less were Nice or Great