Don't use power armor in pvp, it's silly when there are armor penetration perks, items and weapon mods that can possible be stacked to over 100% penetration, like 75% from melee perk, 50% from the anti-armor legendary effect plus 10% armor penetration from certain weapon mods. It would be kind of weird if these didn't stack because then we'd have either a useless perk, useless legendary effect or useless weapon mods in the game. It might look like you are decked in full power armor when doing pvp actually you're just the emperor with new clothes. On top of that your opponent will gain an advantage against you for every piece of useful legendary armor they are using against you, a single -15% dmg from humans is pretty significant already.

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If a higher lvl player goes to challenge a much lower level player but uses far worse gear compared to the low level player, he probably should get his ass kicked for using the worst possible armor for that job :P. Some items are good for pve, others for pvp. Not everyone is used to this but it's pretty standard in many online rpg's that have pvp (like WoW and resilience stat). Just as the gun that does +50% dmg in pvp doesn't help much in pve, neither will the items that are the best for end-game pve do good vs. players using gear that actually benefits them in pvp, no matter what level they are.

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In the time it takes you to reach lvl 68, you should be able to obtain more pvp gear compared to someone who has just reached 31. I know the stash is a bit limited, but one really should make room for a full set of pvp legendary armor+weapon if they wish to pvp. If only to not be against a disadvantage vs. low level players wearing random legendaries just for leveling that happen to have some pvp bonuses.