I am too a solo dude, playing the entire game solo. I also felt it was very hard in the beginning, but now it’s much more fun. So yeah, the game will become easier once you learn the quirks of it. Here are a few suggestions:
(1) The runners (look like dogs)
Use a long distance rifle with a scope and aim for that grey cylinder on top of them. This is their fuel cell and will make the whole thing go boom. Once you get a scope, you will start to have one-shot kills frequently even before they get in range for their melee jumps. If they DO come in close, switch to an assault rifle, SMG or maybe shotgun and just blast’em. They will go down after a few hits.
(2) The hunters (bidepals, run fast and have an aggressive AI)
These are the hardest for me actually, and made me frustrated at the beginning of the game. In the early-game, hide inside buildings but if they come too close or even enter the building, you have to be mobile and run constantly. Out in the open, run sideways in large circles and land a few shots in between. Use flares and fireworks to constantly distract them. This is very useful against hunters. Once you get a better rifle with a scope, aim for their weapons: on top of the robot (on their “shoulder”) and in the right “hand” of the robot (from your viewpoint). If you take out its weapons it will be less harmful, but you still need to pepper it with bullets to make it go down eventually. Once you get the rocket launcher this tactic can be changed for a more traditional anti-tank manoeuvre.
(3) Tanks and Harvester (the really big MFers)
Use EMP devices (I find the small and medium size the best ones as those are throwables, the large one you can only plant on the ground). Now, these big dudes are powerful but SLOW to react, and you can surprise/ambush them by sneaking close until the alert-indicator turns yellow, at which point you start sprinting straight toward them. Throw an EMP at them when in range (throws like a grenade), and plant either a gas canister, scavenged fuel-cell or a land-mine right at their feet when they’re still out cold. Run back again, equip a scoped weapon and when you’re in safe explosive distance, shoot the item you planted at their feet and enjoy the fireworks. If the robot didn’t go down at the first attempt rinse-repeat.
Tactics
Many times several types of robots (especially runners and hunters) appear together. Don’t just run straight in and light the place up - you will die if you do so. Rather, crouch down and observe with binoculars for a bit. Stay undetected.
Do you have a plain field anywhere nearby? A dense forest? High-ground or Low-ground? Water?
In larger fights, you need space to retreat to, or a large circumference you can run around in. Staying inside buildings will only help you sometimes and I wouldn’t count on that as a useful tactic.
I didn’t mention Radios and Boomboxes, but these can be useful to draw the enemies to a certain spot on the map.
Example: I place a radio and a gas canister at the same place on an open-ish area while still undetected. I run quickly into the bushes and wait for the enemies to run toward the sound. When they are all close, my rifle fires at the canister and goes boom, hopefully taking out several enemies at once.
If you don’t have radios or flares: Take out the weaker enemies first. Surprise one runner with a one-shot kill on their fuel-cell as described above. The hunters will be alerted and come for you, so you need to constantly move around and aim for their weapons to make them weaker.
If there is a tank/harvester nearby, be extra careful in taking out the weaker enemies first. It is way easier to take down a tank when it’s just 1v1.
Scavenge/Loot items
Always check dead robots and loot boxes. Pick up gas canisters, grenades (I don’t care for smoke but some use them to cover their tracks, personally I don’t care for those), mines, flares and fireworks. The large fuel-cells and gas canisters (red or white) are excellent for taking down tanks and harvesters. Flairs and fireworks easily distract hunters and runners.
Critical hit zones
So, robots are extra sensitive at certain places. There are several components that will take down robots more easily, but as a starter, aim for (1) Runners: Fuel cell cylinders on their top/back, (2) Hunters: Top of shoulder and right-hand weapons AND their “face” with red flowing dot-lights.
Flying drones
These will detect you from very far away and when they hit their sound horn, every enemy in the vicinity will be alerted to your presence. You need to act when you hear this sound: run away or prepare for a fight. You can take them out before they hit their alarm IF you use a scoped rifle and take them out quickly, or if you hit their horns on the top of the drone. Damaging this component on them will make them unable to alert other robots.
Flares/Radio
Use these distractions to your advantage. You can also use sticky flares and throw them at an enemy, which will make other enemies fire at them. Its good fun to observe sometimes! You can also scavenge the loud horn from the drones sometimes, and when you plant them they will act as a powerful radio alerting all enemies nearby. As before, plant mines and gas canisters at the site and when robots are close, light the place up.