2: Elements - 2
13: The Gap - 2
14: The Descent - 13
16: Anomaly - 6
20: The Decision - 3
22: The Chain - 5
25: The Scapegoat - 1
28: Antipole - 1
30: Maintenance - 2
31: Semiramis Maze - 2
33: Leap of Faith - 5

Most of the lava throwers are hard to miss, see following sections for a few less obvious ones.

First lava thrower is out in the open, second one is located in the secret area with the painting.

At the start of the level two lava throwers are placed behind you. Others - between you and the exit.

All six lava throwers are in the second room.
Make sure that all of them fired before exiting the level, especially the ones closest to exit.

As with 16: Anomaly, make sure that all three lava throwers actually fired or they won't count. Might be tricky with all those bush fires around, but you don't actually have to finish the level after it was counted.