Walk down and notice the hexagonal ledges in the lava pool. You have to jump on them in order to get across.

Start running on the first group of ledges and stop on the last one. Do not stop on the previous ledges because they sink into the lava.

There is another group of six ledges ahead. Jump on the right hand ones and move to the right, as one from the left ledges sinks.

You have to be quick in the next two jumps. The fireballs are coming from everywhere and if one hits you, you lose health. Jump on the next set of ledges ahead and...

... then on the three ledges to the left. Then move to the right.

From there, jump to the right hand group of ledges and then directly to the stairs. If the fireballs prevent you from jumping on the last group of ledges, take some steps backwards and perform a running jump, turning left in midair, to land on the ledge exactly in front of the stairs.