When you run into shielded opponents in the next part of the location, remember when you fought against them at the copper mill. Shooting at them is ineffective. You need to approach them, dodge at the right moment and counter their attack, by pressing Y button at the right moment.
The only enemies that you can get from behind, stealthily, are the three in the screenshot above. Try to seize the opportunity and finish them off with a poison arrow.
At the end of this stage, you are up for a fight with lots of opponents. Try to fire in short bursts of 2-3 shots to their heads. Take cover behind a pillar and make sure that you do not let the opponents with knives anywhere near you.