For focused spammers, you need to keep them at the edge of your view range where they can't see you, so you might have to do lots of backing away if they try and press the attack (so make sure you have the space to do that). Dodge back and forth around their bullets and aim to land blows on the side of the tank which isn't protected. Often, it's good to use surprise to do this so don't shoot all the time, go quiet (so they can't track you) and slip around them, then unleash a volley of shots onto them while still turning around them. Due to the surprise and speed and number of bullets most focused spammers can't block them all if they keep coming in from different directions - never stay in one place unless you can actually blast through the spammer (which you usually can't unless they've built badly). If they get a fix on your position move away while weaving, trying to block their bullets with your own if necessary (sniper can actually do this pretty well), and if they keep chasing you slip shots in on their flanks - while a focused spammer pushes forward, it's sides are more vulnerable because it's moving forwards protected only from the front by its bullets, whereas on a defensive stance it can fall back and take it's time to block your bullets (which is why you should aim for surprise in that case).
Area spammers, basically just blast them till they die:
Octotanks - as long as you keep your aim steady you should be able to cut through an octo with absolutely no problem, apart from the hurricane (low bullet speed, high movement) builds where you need to be more patient because a lot of your bullets will be blocked. Try trapping them between multiple tanks and killing them as they're forced to move towards you, aiming to hit them just to the side of straight on. Spreadshots - you can cut through these too, but their small bullets can also slip through and hurt a lot, so prioritise on blocking nearby spread bullets with your own rather than aiming at it (and dodge where possible as well), as long as you keep a rain of bullets in the correct general direction it will definitely annoy them. With a bit of practice, you'll be able to 1v1 these fine as well, killing them while taking almost no damage Pentashots - Oh these are just horrible. Basically, it's the same 1v1 style - dodge and block first, then aim, but much more difficult because Penta is too strong. Try to aim to land your shots between their cannons if they're keeping their cannons still (so your bullets slip down between their bullet streams)
With all area spammers, just don't get trapped in a small space, it means death (although that applies vs every tank with a sniper), but it's really dangerous against area spammers. Make sure you have space around you because if something else comes up behind you while you're fighting an area spammer in a 1v1, it cuts of about half your screen and as soon as you stop shooting they press forward to try and kill you which they can do without seeing you, so never ever get into a 1v2 with an area spammer.
Drone classes: You need to have max movement really to have a chance
Overlord - Just always aim straight at the body of the incoming overlord, never at the drones. Weave slightly so you get a line of sight on the overlord while running away if possible, and never let an overlord trap you in a small space or you’re instadead. As much as possible look to back away from each drone attack, and get a line of sight onto the overlord body.
Necromancer - Run away while shooting down the drones basically. Be very wary of being unable to back off again, because necro will crush you into anything easily if you can't run away enough while destroying drones. Be careful of a pro surprising you with either of these tanks, that's the one thing I don't really know how to handle well with sniper, and usually a pro overlord surprising me vertically means my death because I can't back away fast enough to get enough space to get a line of sight onto the overlord, and there's no time to destroy drones before the overlord overwhelms you. The second part of this gif of me fighting an overlord might help, "Lucy" is an overlord I've seen get 1m before:
Factory - Similar to overlord, but it's tougher if the factory is pro because they keep their drones close. A 7 movement speed factory that can keep up with you can be fatal. Try to trap them between other players, which they really struggle to deal with, or just abuse any lack of drone control, trying to get a line of sight onto the factory's hull
Battleship - Just shoot at the direction the bs drones are coming from and you'll usually get an offscreen kill. Being able to get offscreen kills is really useful for sniper btw, try to visualise what your opponent is doing outside your view range so you can bombard them with your fast, powerful bullets from off-screen. Shoot your bullets in patterns that means that it's likely something is going to hit, don't just keep shooting in the same direction.