The Division is a game built primarily for multiplayer, which is all well and good unless you're someone who doesn't particularly enjoy playing games with other people – which is an understandable stance given the high quotient of objectionable people in the general population. Although matchmaking for missions is advisable (and team chat is usually off so it's like playing with some marginally more intelligent AI) you can still successfully play The Division by yourself, it's just a bit harder. So here's a guide to make it easier.
This is going to be your main concern if you're playing alone. In lieu of playing with actual people your skill loadout basically functions as your squad, so it's worth experimenting with different ones to find the combination that suits you best (and we have a complete guide to all the medical, tech, and security skills for you to look over).
Stacking both skill slots for damage (say, with the Turret and the Seeker Mine) can take down enemies quickly and efficiently, and the Turret especially is good to have as a second gun. On the other hand this leaves you without med support, so you'd better hope that you can take everyone out before you run out of medkits. You can build for high defense and use security skills, but it'll probably take you longer to finish the fight. It's really a matter of personal taste, but it's something you need to get sorted.
The Division, being part RPG as it is, scales the difficulty of the different areas of New York as you progress through the game. Try to stay a couple of levels above the recommended threshold for the area you're in (which will show up on the map if you highlight a district or a particular mission). This will give you a bit of an advantage in the bigger fights, but without nerfing it so much in your favour that it becomes boring. You can sweep up a ton of XP to level up by doing the side missions and encounters, as well as picking up collectibles like phones. Clearing an area also has the added bonus of reducing some of the enemy activity in it.
This is really something you should do in The Division in general, but if you're playing by yourself it's even more important. Don't sprint into new areas, in case you run headlong into a group of gang members. Use your aiming reticule if you're not sure if a figure ahead of you is an enemy or a civilian – it'll turn red if they're a valid target, and you can prepare accordingly. Go into cover well before you need to when approaching enemies, especially if they haven't seen you coming, and then move up staying in cover to get the drop on them. If you plan for the worst then the worst should, theoretically, be less likely to happen.
When you're out on your own it means nobody is around to watch your back, so you need to make sure you're not getting flanked or surrounded while your fire is being drawn by one particular enemy. Pulse is a really useful Skill, but if you're flying solo it's probably not one you can afford to have in your loadout so you're going to have to remain especially vigilant with your actual eyes and watch the minimap to roughly track enemy movements. This is especially important in higher level areas, where taking a few rounds in the back can mean the difference between sweet victory and respawning at a safe house miles away.
It's automatically harder for enemies to flank your position if you're not in that position any more. As much as you can without opening yourself up to fire, you should move to a different bit of cover when the opportunity comes up. Getting pinned down in one place makes you more vulnerable and easier to surround, while moving makes you a harder target and gives you new angles of attack on enemies.
The dude dug in behind cover firing at you every now and then? Not a liability. The dude running at you with an axe or a shotgun? He's more of an immediate concern.Take out the enemies you know are going to be more dangerous to you first, so think about the ones lobbing grenades, or Cleaners close enough to start using their flamethrower. Keep an extra sharp eye out for snipers, and try and move to cover that will put you out of their line of fire completely. That way you can deal with other threats without having to worry about their pot shots.
Fewer guns = not as many bullets being fired, which in turn means you're going to use up more of your own ammo supply per bad guy than you would in a team. If you're playing solo at a higher level you'll be in danger of running out of ammo on your favourite weapons, since enemies will absorb more damage and running out to grab ammo drops is harder without a team giving covering fire, so conserve it as far as possible.
Try for as many headshots as you can (the One Is None perk is good for this), and aim for weak spots on enemies that have them. If you're using the Turret skill then throw it on the top of cars and other elevated cover to give it as much range as possible on hostile targets, and don't forget that your side arm has unlimited ammo, so get a decent pistol and switch to it when an enemy is on low health to finish them off. Every little helps.
You know what else helps with ammo? Having a few different weapon types. When you use a restock box you'll be resupplied with ammo for every different weapon type in your inventory, so if you're really down to the wire in a fight you can take cover and quickly equip a different weapon (although this is not recommended as a first recourse, because it's a really risky thing to do in the middle of getting shot at). Having a mix of short, mid, and long range weapons to choose from means you're also able to adapt to different situations appropriately. Making sure you always have a marksman rifle to lands on, for example, is particularly useful against snipers and Cleaners.
When all you have to rely on is yourself then you'd better make sure that that self is as well kitted out as it can be. This is especially important in The Division, where your gear changes your primary stats. Paying attention to the scores on what you're wearing, as well as any bonuses attached, can mean the difference between life and death when you're on your own. Don't skimp, or try and slide by with a couple of pieces of lower level stuff until you get some new drops. If you have to buy some better gear, then do it. Nobody likes a cheapskate. Especially not a dead one.