Witch is all about avoiding, flying around on your broomstick, controlling enemies, and smacking things with a giant flyswatter.
I actually really liked the one I played for PvP, because the amount of annoyance points you can rack up before an enemy gets their stun lock off on you is amazing.
You mean people do PvP with characters other than gunners? There's nothing like getting locked on the ground because they can shoot down at you while you throw punches way above their head because they're so frigging lanky.
Maybe monks/I just suck at PvP, but I've never once won a match.
Yeah, PvP is surprisingly hard, especially if you don't have a good connection (like me), and gunners are very good at it. Monks are also very good at it, but they have the notable disadvantage on gunners of not having a ranged attack. All you can do is remember you can move up and down too (surprisingly easy to forget in the heat of battle), and walk around their bullets, and use dash (It has tons of invincibility frames) to get in close to them. Sway is also of use to get out of a sticky situation. Sometimes it'll make you invincible log enough for their combo/skill to time out and leave them open for a second.
Of note is the fact that monks main bonus in PvP is that it's very easy to get a huge chain going that will just end them entirely if you can get your hands on them. This'll take some experimenting with skills (if you're a lower level monk, 20-25 you prolly don't have enough skills to pull it off quite yet, I'd wait until you get access to sway and machinegun jab at least), but you can definitely stunlock dudes, if you can get close enough for it.
PvP is also like PvP in every other game in that people that are much better at it then any normal human can be expected to be are constantly roving around and kicking teeth in.