Use the grapple ring off to the left to get up onto the ledge above the spiked wall.
There's another grapple ring above that will enable you to reach the alcove with the red skull (1/5).
Light the gas vent in the corner to the right. This heats the pool below, raising another pillar and opening another gate. As the next 2 enemies approach, you can wait for them to come to you, or if you haven't beaten the spike wall challenge, quickly jump down onto the ledge near the spikes and run behind the column on the right.
The warriors' bombs can't hit you here, and you'll be in a good spot to fight them for the challenge.
Shoot the first warrior until he falls down.
Then try and blast his body into the spikes with a remote bomb. Again, place the bomb so the edge of the blast radius just covers the enemy's body. If it's too close, the bomb will kill him instead of the spikes.
If you succeed, you'll see a message like this. If it doesn't work, try the same thing with the next enemy.
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