Abandoned Mines - First Combat in the Ruined Chancel with Shallow Pools

Move stealthily into the room on the left...

...and take cover behind the nearest low wall. There are 6 enemies ahead, but if you're patient, you can take them out one or two at a time.

As you crouch below the glass jar sitting on the wall...

...a pop-up explains how to craft SMOKE GRENADES. Grab the jar and hold RB/Middle Mouse to combine it with cloth from your inventory.

Aim in the vicinity of the first enemy and throw the grenade just like other projectiles.

Smoke grenades don't kill, but they do a great job incapacitating enemies so you can finish them off with an arrow or stealth attack.

If none of the other enemies notice this—and they didn't when I tried it—move forward again...

...wait for the smoke to clear, and then target second man standing near the far wall with his back toward Lara. He's wearing a helmet, so a bow headshot won't work, but you can use a poison arrow or shoot him once with a charged shot and then follow up with another arrow or two, killing him before he can alert his buddies. (Or use a gun equipped with a suppressor.)

When he's dead, move around to the small hole in the crumbling wall between you and the other men outside.

Aim through this hole and use a poison arrow to take out the helmeted enemy near the anti-aircraft gun.

Now move around to the wide doorway leading outside.

There are other low walls here where you can hide and craft molotov cocktails (and/or exploding gas cans if you've unlocked the Incendiary Bombs skill).

There's one enemy directly ahead, and 2 more men come out of the alcove on the left to see what happened to the guy manning the big gun. Try to throw a firebomb while they're still clustered together.

You can kill two or possibly all three this way.

Quickly finish off any survivors, who are now fully aware of your presence.