I really need to go back and finish Platinum sometime. Let's see, I was in Sunyshore waiting to challenge Volkner's Gym ... come to think of it, I never even entered his Gym in the first place.
And Mesprit is fast. But at least Mesprit is a roamer, not a defensive tank like Uxie was or an Nasty Plot powerhouse like Azelf was (Ugh...) And the best part about G4 Roamers is if you put them to sleep they'll still flee, which means they keep the Sleep status indefinitely and you can just start spamming Quick Balls as you hunt them down.
So the battle with Volkner ... well, my Gabite took out his Jolteon (after some effort, got paralyzed by a Body Slam), then my Lv.48 female Rivalry Luxray (barely) takes out his Lv.48 male Rivalry Luxray (yeah, two equal lions Crunching each other to death -- but mine's faster), Infernape gets lucky enough to down his Electivire in two hits (Flame Wheel critical + Close Combat), and then finishes off his Raichu in one. Wasn't terribly proud of having to use a few healing items, but the Gym Leaders have some items themselves, and a win's still a win, right?