This is a guide on the best possible ways to kill each machine under nearly any circumstances. Tanks and Harvesters are not included, however, because I have yet to encounter any. All of the tactics inlcuded should be able to take out the enemy in one shot/pass, with minimal damage to you.
Ticks can be quite dangerous if you cannot deal with them properly. Whenever you know you are in an area with lots of ticks, have your pistol or other small handgun at the ready. When you see one, you simply must not jump or react, just shoot it once in the very center of it's body before it begins to move or even register you are there. They will instantly blow up. Doing this will save ammo to your other guns which you need for bigger baddies. Plus, if you're invading a bunker, they will provide a nice light source for your buddy who forgot to pick up the flashlight at the beginning of the game.
Runners, AKA Dogs. The best way to take them out as I have seen is to snipe the gas tank on top of them with a hunting rifle or sniper rifle, or if you're extremely skilled and extremely lucky, an assault rifle on semi-automatic. Doing that will instantly blow them up. For whatever reason the other runners will not care at all that one of them spontaneously combusted right before them and they will not detect your presence. Alternatively, if they detect you before you shoot or they are in very close range to you, shoot the tank with your pistol or other hand gun.
Some of the military, higher-level runners have armor on the front of them because they are big brain and they know what you're doing. In that case, either shoot them like I told you but from the side or back, or just dash up about 3 feet away from them and blast them with a shotgun, and then run back to your original location. This next tip is not always very effective, but I have found that in the event of being swarmed you can just do a spray & pray with an SMG and do a slow, horizontal wave of bullets while still targeting the tank.
Seekers, also known as drones. For some reason they are incredibly difficult to kill and will just soak up bullets which makes no sense, so in order to kill them you have to snipe one of their little flying engines. It's as simple as that. I think if you have a powerful enough gun you can blast of it's yodeling device and it will then become useless. Shooting the drone anywhere else other than those two locations will do essentially nothing unless you stand there blasting it for about 50 seconds.
Hunters, also known as Big Sword Thing, and Big Boi. They can be extremely difficult to defeat and will often screw you over easily as they are faster than you (including you on a bike) and can take out your entire health bar with a well-placed attack. However, you can also take them out with a well-placed attack.
There are two ways I have killed these things other than just shooting: First, you throw a flare or some other distracting thing behind them, they turn around, and then just empty your entire SMG on them multiple times. The SMG is INCREDIBLY effective against Hunters. Secondly, this one is a bit more strategic. Run to the halfway point between you and Hunter and place a gas canister, then run back to long range. Shoot off the Hunter's gun which forces him into Melee mode. He will charge at you, so shoot the gas canister as he runs over it. Kaboom.