What stats do you want your pokemon trained in? You can max out two stats.
You probably want attack for blaziken, slaking, and metagross, go to the town near the desert and battle machops. I usually cheat to make every pokemon the wild a heracross though, because whenever I wanna battle a machop I keep running into numels and if I want to battle a numel I keep getting machops.
Use the Help Thread Trainer's Checklist (Look towards the end of Post 6 on Page 1) and you'll find a good list of which pokemon and their location to fight for each stat you are EV training in.
BTW, there are two lists: one for Ruby, and one for Sapphire. You may want to take a look.
EV training help!
