Whatever you're doing, you need an escape route, so don't put your back against a wall, have it against somewhere you can escape to safety. Levels are laid out with obstacles like crates everywhere, which you can dash behind repeatedly to avoid bullets and hopefully get the chance to regain your health. Whether running about the stage or lying in wait for enemies, make sure you know exactly where you're going to go if you get spotted first. The portals which jump you from location to location are great for this.
Hunt or hide
The eternal question, right? Whichever option you choose, you want to be aware of your surroundings. When hunting, make sure no one is ever following you (listen for gunshots to be safe) and make sure you have a mental image of where you can rush back to safety if things go wrong. Hiding is possibly more optimal. You can place down an automatic turret while lurking. The turret will keep an eye on what's in front, while you can keep checking your back. As soon as the turret fires, you've found your enemy.
Precise aim
Tapping on the screen twice with your right thumb allows you to aim down sights, which is of course very useful, and definitely the best way to go for rifles and other weapons. Shotguns, however, will do more damage up close. Interestingly, aiming at your opponent while moving towards them will slow you to walk speed instead of sprint speed so you can hipfire - not always optimal. If you want to get closer with any weapon, don't aim at your enemy. Aim at the side of them, and then aim at them when ready to fire. Best tactic to use with the shotgun, though it can help with the ranges of other weapons too.
The stealthy approach
Running head on once you see an enemy is certainly one way to do things, but for my money you should instead be predicting where they'll go next. Map layouts are pretty straightforward so you won't struggle too much to memorise their general shape, and if you see an enemy run in one direction, you'll know which direction to head in yourself in order to head them off. Stick behind cover and don't run about too obviously, and you'll likely get the jump on your enemies. Well, so you hope.
Well stocked
When you jump into a PVP match your primary ammo should be well stocked, but your secondary ammo will be empty. Pistol ammo is always infinite, though. Your secondary is likely your more powerful weapon, though. Sniper rifles, rockets and more are secondaries, and ammo must be pried from slowly regenerating ammo crates around the stages. Make sure to be careful when shooting with this ammo, and use whatever gun is more plentiful if you're unsure of the accuracy.