I don’t know what’s up with the tryhard outfits. My best guess is that they know it’s annoying to have to look upon something that stupid, and a big part of their joy is in annoying other players.
As for tactics, sure, they could be bullsharking. But chances are, they’ve just gotten better at the head-shot. If you’re using auto-aim, it locks onto the torso. What you need to do is learn to bump it up quickly so you get the head-shot. If you’re not wearing a bullet-proof helmet, or equivilent, it only takes one bullet to kill you. Three shots, if you have the helmet. If they’re hitting your head while you’re pounding on their chest, you’re gonna go down a lot faster than they are.
There are a lot of other street-fighting techniques that are helpful. For instance, when and how to use combat rolls. It’s usually better not to be the first one to roll. What you want to do is, when they roll, you roll in the opposite direction. When they come up, you’ll still be in your roll, so they haven’t got the lock on you. But when you come up, you’ll lock onto them immediately.
You want to avoid using cover. That is, you don’t want to be locked onto a wall in most cases because that just prevents you from moving quickly. Better to step quickly back and forth from behind the wall, taking your shots. Also, if you see an RPG headed your way, (and we all know tryhards will resort to RPGs), you want to be able move away quickly. Having to unlock yourself from the wall, even if it goes well and you don’t get stuck to something else, adds an extra step you don’t have time for.
It’s worth learning some tryhard tricks. As they say, all’s fair in love and war, and you’re only kidding yourself if you start thinking this or that way to fight is “honorable” or “fair”. People are never going to fight the way you think they should fight, so just forget about that. Moral superiority only gets you killed. Learn about how they can bullshark for thirty minutes straight, and keep the cops off them as well. You might need it sometime because, if nothing else, it can prevent your opponent from using it. Learn which Mk II ammo is best for what purpose, and have one or two guns loaded with it. Keep some other guns with regular ammo. That way, you can use bigger clips, and never worry about running out of ammo. It’s unfortunate that we can’t change clips in the field, so you’re kinda stuck with exploding ammo on the Heavy Sniper, (unless you’re a very good shot - you can take down a jet with FMJ rounds, but it’s not easy). It would also be nice if we could upgrade the regular sniper rifle. Either option would’ve been better than nerfing the exploding ammo, IMHO.
Go on YouTube and do a search for “GTA how to street fight combat PvP”, or try some other search terms. Some videos are better than others, but you’ll certainly pick up some tips along the way. Then, practice makes perfect!