If you had no clue what Ev's your opponent had, then what are you trying to get at? If he gave it speed evs, he wouldn't give it defense evs, or else sp att would be severely handicapped. If he had not given it defense evs, it would be OHKO'd. It was not, and I thus surmised that there was a high chance it would be slow. Anyway, if he had given the evs to speed too, sp att wouldn't be quite as effective as before against other pokes. I know what I'm doing.Your post was extremely messy and hard to understand but from what I can see, it sounds like Magneton comes in on a switch and faints because you know it was coming =\.
Came in on a switch, but didn't faint because it was tanked in defense, but that would either not allow it to be pumped in sp att or spd. If it's sp att, that is a bad magneton. It wouldn't be able to do much else to other pokes. If it wasn't speed, since I made Skarm have 4 spd evs, it would not outrun even if he had given the last 6 after dividing 252/252 to speed, since you said that equal speed is decided by who came out first. Eitherway, what I'm trying to say is that unless the opponent knows my tricks, it's a sure-kill way of magneton. Also, HP ground isn't just for magneton you know. I can catch Jolteon on the switch, attack some steel types that try to up speed or attack to sweep with super effective damage (metagross) and overall, I find it quite useful, attacking skarm...
Like I said earlier, equal speed is decided by who was out first. If your Skarm was out and Magnaton is switch in after and both have maxed out speed, you will win. Same thing backward, I have done it more than enough times with lots of Pokemon. My HP Ice Typloison alone OHKOed more than its fair share of Salamences because it was out first.
