What's the best way to make your Pokemon stronger and when is the best time to give them stat building items?
Yes, that process is called EV training
Anyways, any level is fine, but preferably at higher levels. Look enter link description here and use one of the high levels and EV train in that level.
The best way to train your Pokemon is for first to not let them have a single battle until level 100 (Unless you have some hoenn berries on you) as What you are referring to is none over then EV training. Not battling til level 100 helps to make sure you know you aren't wasting EV's anywhere and they are also worth more at Level 100.Go here if you want to get a better explanation on EV training.
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