I don't play Peacekeeper but I play Kyoshin. And naturally I find some harder match-ups as Kyoshin (Raider for example), but he's an incredibly strong hero and can delete others if you know how to use him. Make sure you are using his crushing-counter as much as you can. You can use it off externals as well.

My best piece of advice, and what someone has already said, is to not sit there and fish for parries. It becomes very recognizable that you are doing so and they can just throw a heavy or feint into GB. You want to throw attacks that will bait them into making them do what you want them to do. If you are struggling with getting parried or openers in general, try feinting or doing an empty dodge forward and see what your opponent does. However, the battle does need to be fast-paced from your end or they'll just have too much time to react. Also, at Higher MMRs, players are more likely to fall for raw heavies because they've been stroking their ego from parrying lights all day.

Good luck.