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A stamina meter.
More powerful Pokémon will tire after being out for so long, a la the Pokémon Trainer frim SSBB. Tiring will progressively make them slower and weaker before they must switch out to rest, when they'll need to stay in reserve for a couple of turns until their meter can go down. Using more powerful attacks will make the meter go down much faster, so will recovery and some status moves such as Protect or Toxic. For instance, a powerful move backed with an amount of attack boosts may warrant a guaranteed kill, but will deplete stamina almost completely, so 'sweeping' is out of the picture. Completely fatigued Pokémon can fall into Confusion or even Sleep, so it's worth avoiding. Also switching too often can also serve to tire out your Pokemon. Lower levelled Pokémon aren't affected by tiring as much, so they function relatively the same at the games beginning.
