I do actually walk too, but when I'm at home, I'll just leave the walker on a fan, tiny vibrations are enough to set it off.
It's pretty simple, ii's are either group A or B, iii's are group A, i are obviously group C.
Group A can either be a surfing or a flying pikachu, group B can be a volt tackle or a flail pikachu. Get to 5k steps, catch a ii pikachu, send it through gift to DS, and see what it is, or get to 10k, catch a couple of iiis and send it through gift.
As far as I've experienced, when you send your pokemon to DS through gift, it doesn't reset your choice. When you go out on a stroll, the system automatically picks 1 pokemon from group A, one from group B, and one from group C. That means until you come back from a stroll and go back out, you're stuck with the one the game assigned to you.
So like I said, get to 5k steps, send to ds through gift. See if it's a volt tackle one. Get to 10k, send to ds through gift, see which one it is. When you get the one you want, don't end the stroll until you get like 3.
When you get the 5k, or 10k, and are going to send to ds, it resets your watts. So becareful if that's a big deal to you.
Also the more steps the more likely to have iis and iiis, at 40k (what I usually have after either a night on the fan or a day walking), I get one like every other minute.
When you get a i, or a ii you don't want. Just keep mashing evade, it let's you finish the battle quickly.
Good luck! :)