When I play alone, my first objective above all of them is to locate the enemy. After that comes remaining undetected. I always find in games like this it's an easy kill if you set traps or ambushes for the enemy. Typically I either find cover in a place away from the objective, sometimes so I can see past a corner of their likely position.
But, as spotting the enemy is my first objective, sometimes I will compromise my position to spot them (I.E. sprinting out of an ally and into the street for a quick glance), then lure them into an area I can fight them in much easier, or even ambush.
By the way...pay attention to that little radar at the bottom. You can see the enemies position if they are in a car or shooting. As a specific example, I've used this to my advantage by training my sights on a steel door I thought they would go through, and the kicking animation leaves them immobile for just a second, but it's a battle winning second.
Anyone else want to throw out their approach to solo combat?