I get to the bit of lava, and can't figure out what to do! I've tried shooting gooballs from the tail up through the flame, but they just bounce off of the cave wall and into the lava.
Help!

When shooting goo balls from the strand on the left, try getting the strand on the right to start swinging. The new angle will give you an easier shot through the fire.

You just have to ignite the string dangling down towards the lava. For this, the string has to touch the torch. No Goo ball needs to be burning for this, at least not necessarily. I think you get the point.

Yeah, I've done it NightFright's way too, although I must admit I've always found my way to be easier. Still, to each his own. Go with whatever works for you.