After the short cut scene in which Sam escapes from the Solarii, move to the corner of the roof and slide down the zip line into a shack.

Here you'll find more rifle/SMG ammo and 2 crates of grenades.

Stand back at a safe distance and use a grenade to destroy the metal barrier blocking the doorway.

If you then step slowly into the doorway with bow or rifle drawn, you can take out 3 enemies who appear across the street. Shoot the one standing below first.

Then wait for the others to appear on the upper level. One of them comes down toward you; the other shoots from the balcony. Stay behind the wall while he's firing. Then shoot him when he pauses to reload. Lara is safe for now.

Drop down into the room below and retrieve more grenades and rifle/SMG ammo. (You can also drop down here and use this wall for cover as you fight the first 3 enemies.)

As you move out into the street, more enemies emerge. Again, there's no one correct way of handling this battle. Here's one possible strategy:

When you first move out into the street, take cover behind the 2 metal crates. There are 2 guys in the building ahead on the left side of the street—one on the balcony and one below and to the left.

Aim your rifle just above the head of the one on the balcony and fire a grenade.

This should take care of both of them.

Next, deal with the sniper who appears above on your left, on the upper floor of the building with the 2 doorways at ground level.

If you don't kill these 3 quickly and they start throwing dynamite or Molotov cocktails while Lara is crouched behind the crates...

...scramble into the room with the 2 doorways on the left.

Here you'll find rifle/SMG ammo and grenades. Then you can either use the metal crates in the doorway for cover as you continue fighting or return to the crates in the middle of the street.