All Mega Pokemon cannot stay evolved after the battle. Also, only one Pokemon can be Mega Evolved at a time.
Even though Mega Garchomp does not receive enough usage to be in the Overused (OU) tier, it resides in the tier due to its base form receiving enough usage to keep it from dropping. Mega Garchomp plays the role of wallbreaker whether it be through brute force after gaining Swords Dance boosts or with its good mixed attacking stats. It has the benefit of having more Special Attack but is burdened by the drop in Speed and the fact that it is a Mega Evolution when there are many other Mega Pokemon with more to bring to a team.
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