When I tried to do this, when I waited on the geyser like the other solution suggests, it NEVER erupted. So what I did, I pressed
and used the 'double jump' mutator with nothing else.
Stand to the edge of the geyser. It will explode every 2 to 4 seconds.. It'll take a couple of attempts, but use the double jump to jump over the geyser's spout when you think it's going to erupt. Make sure you don't touch the sandy area around the spout or it'll cancel the upcoming eruption.
The geyser itself is at the south west of the map, just south south east of the firey castle area.