After you open the gate in the Fire Temple, you break the rocks in the ceiling and reveal Marbled Gohma.
Start the Battle
If you haven't already, go to a wall where there is a large grey stone. Launch Yunobo up the wall here and he will be able to roll to the rocks in the ceiling and break them. Then the battle will begin.
Defeat Marbled Gohma
Marbled Gohma will try to stomp you with one of its legs. Run away to avoid getting hit.
Have Yunobo hit one of Marbled Gohma's legs, and it will drop some explosive boulders near you. Run away from them to avoid their explosions.
Continue having Yunobo hit Marbled Gohma's legs until Gohma collapses. Gohma's body creates a ramp that you can go up, allowing you run up to Gohma's eye and hit it repeatedly. You might want to jump off when Gohma starts to get up, because you will take a small amount of damage if Gohma gets up before you jump off.
When Gohma gets up, its legs are temporarily invulnerable, so wait all four legs have reappeared before you try to hit them.
When Gohma's health reaches 50%, there will be a cutscene, and Gohma will attach itself to the ceiling. You now need to have Yunobo roll up the wall to hit Gohma's legs. You will also need to avoid the much larger explosive boulders that Gohma tries to surround you with.
When Yunobo hits a leg, Gohma will fall, and you can run up its ramp as before and hit its eye repeatedly.
After the Battle
There is a Heart Container for you to take after the battle. Talk to Yunobo and there will be a cutscene.
Afterward, you will be back in Goron City. This completes the quest.
If you have done the quest Impa and the Geoglyphs and started The Dragon's Tears, you will find Impa and Cado nearby. They can tell you more about Geoglyphs that you haven't found yet. Read The Dragon's Tears for more information.
You can also return to Lookout Landing, where you might find some clues about where to go next for the Regional Phenomena quests. See the following links for more information:
While in Lookout Landing, if you have completed Camera Work in the Depths but you haven't started A Mystery in the Depths, you can talk to Josha, and she gives you some Giant Brightbloom Seeds and arrows. This starts the quest A Mystery in the Depths.
Also in Lookout Landing, if you have completed at least two Temples, you can talk to Karson to start the side quest The Incomplete Stable. There is a plank next to Karson that fits perfectly, so use Ultrahand to put it in the gap in the stable roof to complete the quest. After that, you can talk to Lester to use the stable services.
After Regional Phenomena
If you have completed all four Regional Phenomena quests, go to Lookout Landing and climb the ladders to get onto the roof of Purah's lab. Talk to her there, and there will be a cutscene. This completes the Regional Phenomena quest. Your goal now is to enter Hyrule Castle.