Inside the boss chamber, continue forward to notice the boulder from earlier rolling along a path of bones. Further along, Fi will converse with you. Nearby, you can restore your hearts and Bombs – for the former, only take what you need. Further ahead, you’ll watch another scene prior to your boss.
Initially, Scaldera will begin by rolling up the incline; stay off to the side to avoid it. Afterwards, make your way to the top of the incline – there is a supply of hearts and Bomb Flowers are there. Be sure to hit the boss with a bomb to send it to the bottom of the hill (the boss does this automatically by going too far uphill).
Upon recovering, the boss will open its mouth and suck in stuff. throw or roll a Bomb during this time to stun the boss. This also exposes its eye, its weak point, and your target. Hit it a few times. Once Scaldera recovers, dash back uphill.
Scaldera then can shoot several fireballs (just sidestep them), roll back uphill, or do another vacuuming move.
After this, it is rinse and repeat, more or less. The only things worth noting include that Scaldera will eventually go uphill using its legs; let it go uphill, then it’ll roll downhill. Also note the Bomb/heart supplies at the top and bottom of the incline.
After defeating Scaldera, collect the Heart Container and go through doorway atop the incline … after restocking, of course!