Pretty self explanitory. One thing tho. If I attacked it just when it mega evolved, so it megas then I attack, would pressure drop it? Or in like Gen 6 when speed didn't change till next turn, would the ability be like that where the first time I attack(in gen 6) after they mega I get the drop, but not later?(sorry if this is confusing)
The effects of Pressure go away once mega evolved.
Replay
Hope this helps! :)
Generation V onward Pressure now only affects opponents. Wild Pokémon are more likely to call for help when facing a Pokémon with Pressure. It shares this effect with Intimidate and Unnerve
Since Bulbapedia says nothing about Generation 6 or mega evolution, the effect wears off
Source: https://bulbapedia.bulbagarden.net/wiki/Pressure_(Ability)#Generation_V_onward
Hope this helps :)