As you enter the next room, the doorway behind Lara collapses, trapping her here. Scramble behind the rusted mine cart, top up your rifle ammo...
...and then quickly take out the first 2 enemies.
When they die, 3 Trinity soldiers with clubs and metal shields burst into the room. The one on the lower level immediately begins to move toward you while the others throw explosives from above.
You can't penetrate a metal shield with gunfire, but poison arrows work just fine, as do molotovs. If you're quick and you have at least 3 poison arrows, you can quickly take out each of the men before they get too close.
A successful hit with a poison arrow causes the enemy to drop his shield...then drop dead. Repeat this with the other enemies and the fight is over.
- OR -
If you don't have enough poison arrows for all of them, or you just want to hone your combat skills, there are alternatives. The on-screen tutorial explains how to dodge. Wait for the first shielded man to lunge toward you. Then press B/C to dodge...
...and he'll overbalance, leaving himself open to attack. Shoot him as many times as you can. If you don't manage to kill him before he raises his shield again, repeat the process until he falls.
- OR -
If you've unlocked the Dodge Counter skill (Tier I Brawler), the dodge maneuver is a little more complicated but also more powerful. Wait for an enemy to lunge toward you. Then press B/C to dodge.
This triggers a brief QTE. When you see the concentric circles with the Melee icon in the middle, wait for the thin gray ring to contract into the center of the circle. Then press Melee (Y/F).
This will temporarily incapacitate the enemy. You'll see a skull icon over his head. Move in close and press Melee (Y/F) again.
You'll then be prompted to choke him out with your bow by tapping Interact repeatedly. Later, when you unlock the more powerful versions of this skill, Dodge Kill and Dodge Kill Mastery, your dodge attacks will be faster and more lethal. The Duelist Reflexes skill gives you more time to execute the dodge counter and gives bonus XP when you successfully perform a dodge kill. So get to know this technique. It will become very useful later in the game.
When you've taken care of the first guy, a second shielded enemy jumps down to confront you while the third man continues to throw explosives.
Occasionally you'll get lucky and one enemy's grenade will damage the other, but don't count on this.
Either use the dodge maneuver again (as shown above), craft a molotov and throw it at the shielded enemy (not shown)...
...or lure him underneath the gas leak, run away...
...and throw a molotov to ignite the gas. Note that throwing molotovs also burns the wooden boxes you might otherwise use for cover, so use this tactic with caution.
When the second man falls, the third jumps down and attacks. When you defeat him, the exit finally opens.
- OR -
Alternatively, there are gas leaks above each of the elevated positions where the second and third man stand at the beginning of the fight. If you can manage to avoid the first enemy while you craft molotovs and throw them at these gas leaks, you may be able to take out the two men from below. Then finish off the first with the dodge maneuver or another molotov.