I would say that you should aim for at least level 47 or 48 for 3 or 4 Pokemon. And have a strong neutral or Super Effective move to hit them, because weak moves will not be doing that much damage when you are near equal levels.
As for EV training, it's your decision. I beat the whole game without EV training. but then again, my Pokemon were pretty over leveled :P. I would definitely consider it if you have the time, since you will be battling Pokemon near the level of yours.