Such as Sableye, he gains stat changes during his mega evolution. Before he mega evolves his best stats are his attack and defense, but when he mega evolves his best stats are his defense and special defense. So should I super train his mega stats or normal stats?
41 Answer
Ultimately, it depends on how you intend to use the Pokemon. When you "Train Stats" you are gaining Effort Value points. For every 4 EV points, a stat is raised by 1, with a maximum of 252EVs in a single stat (+63), and a maximum 510EVs (+127) across all stats.
So if we take a lvl 100 Sableye: regardless of whether it is mega evolved or not, putting 252 EVs into defense will increase defense by 63. The only thing that changes is its base stats (from 75 to 125)
Whether you decide to train based on a Pokemon's mega-evolution stats or not is going to depend greatly on the Pokemon and how you intend to use it. Some mega-evolutions are not advantageous in all situations and may necessitate not using them immediately due to changes to type, ability, or stats.
Smogon has a great collection of build strategies that are used in competitive play - you don't necessarily have to follow them to the letter, you can alter them to suit your play-style if you want. For what it's worth, their guides on Sableye tend to recommend putting most of your EVs into bolstering the somewhat lackluster HP, and then dividing the rest between Defense and Special Defense. Doing damage by inflicting status conditions and using things like Foul Play to deal damage based on the target's Attack stat.
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