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To maximize Elo gain and minimize Elo lost, players are incentivized to match up against opponents close to player’s own Elo. The laddering opponent searching process will prioritize on opponents close to your Elo, and the longer you stay searching, the server will reach for players “outside...
I had an Ampharos with ability Plus, and mega-Manectric with ability Minus (before mega-evolve). However, the plus and minus don't work upon entering in double battle, not boosting anything.