I'm playing co-op with a friend. It's hard as fuck but we're slowly making progress.
I'm not sure if you can respawn in singleplayer, but I'm guessing you can since the game is somewhat based around it.
>Hit and run
One of the most useful tactics. Remember, a bullet can kill, but so can a car. Running over police/military and stealing their equipment is a good way to get items early on. You can always steal enemy vehicles and fucking floor it back to the garage when you feel like you might need one.
>Use HR
HR won't do you any good if you just hold onto it. Use it to create roadblocks around areas you've conquered, scouts to send into roads where enemies might come from, and to fortify your bases.
It's always good to have a mortar laying around on the borders of your area in case you need it during an attack.
>Know the game
Radio tower and power stations increase your reputation with the towns around them. If you can't take them, destroy them instead, then do missions to get support from towns.
>Cheat to fix bugs
Vehicles with bullet proof glass/armor are literally impossible to steal since you can't shoot through with the available weapons. The AI is supposed to abandon the vehicle when all weapons and wheels are damaged, but for some reason that's bugged for me. A mine and a RPG later and and APC can't even move anymore, but the AI stubbornly stands inside for some reason, so it's only fair to go into Zeus and force them to dismount. You can use Single Player Cheat Menu for that, just don't be a faggot and cheat when not absolutely needed, don't ruin your own fun.
>A mission is a mission, not an order
Missions can sometime be really fucking hard and have little to no rewards. If it looks hard, it probably is hard. Don't push too much.
Destroy missions are good since they can cripple the enemy, as are traitor missions since those will hurt you instead, if you don't complete them, with both of those you can just run in undercover, place a mine and fucking run away.