You have to make sure to complete the wave within the time limit. Also, apparently there is a chance that not enough Raging Fires will even spawn to complete the wave.
"In a few moments, the lava will spread and Raging Fires will spawn. You must kill these and use their bodies on the Crucible to the north-west with the limited time and space you have. For the first wave, you have 60 seconds to kill 6; for the second, you have 50 seconds to kill 5; for the third, you have 40 seconds to kill 4. 30 seconds after killing the third wave you will be teleported to the next area and receive the achievement Go with da Lava Flow.
Note: The number of Raging Fires that spawn is random, so it may happen that not enough of them spawn when all rounds finish. If this happens or if your team fails to use enough bodies on the Crucible in time, you will be sent back to the western teleport in the previous area, through which you came. You'll then have to turn on all Strange Machines of this area again to restart."