Usually you wanna fight low level Pokemon when you EV train so that you can max out whatever you want before they level. That way, when they level they get the maximum amount they can per level after that.
If you change your mind to do that instead beat up Psyducks, Houndours, Slugma and Horseas. :) They'll all only give you 1 EV in Special Attack but you can find pretty low level versions of them.