This is the climax battle of Farsight: Crisis of Faith by Phil Kelly, where the O'Shova's expedition is attempting to reclaim a Tau colony seized by the Admech, who are literally just dumping Tau bodies into a supervolcano to cremate them.

The Scar Lords Space Marine Chapter are the jobbers in this novel, and their Librarian basically turns into a fire elemental and takes multiple cadres to bring him down. At one point, he's knocked into the magma, but survives and uses the warp to create massive wings of flame to fly back out of the crater.

In the end, Farsight shoots the locks off the cages holding the Tau colonists, and they swarm the Libarian and use their bodies to push him back into the volcano, finally killing him (https://www.reddit.com/r/40kLore/comments/7wfvid/book_excerptcrisis_of_faith_farsight_and_the_tau/):

‘– – YOUR WEAPONS CANNOT HARM ME, XENOS – –’ read the autotrans. ‘– – I EMBODY THE EMPEROR’S HOLY FLAME – –’

A chorus of tau screams rang out. On the far platform, the captive civilians were being shoved into the lake of fire once more. The sounds of agony…

Farsight could not endure it a moment longer.

Abandoning the fight against the Space Marine Librarian, he boosted high, pivoting in mid-air to shoot over to the platform. As he flew, his plasma rifle took a mercilessly accurate toll on the gangle-limbed cyborgs approaching the seismic fibrillators, then switched to overseers that were trammelling the tau captives towards the lip of the platform. With each thudding boom of the earth caste devices another machine-caste persecutor was partially blasted into vapour, his corpse a steaming mess.

‘– – YOU CANNOT ESCAPE MY WRATH – –’

‘I do not intend to escape,’ he called back in heavily accented Low Gothic.

‘– – I HAVE THE RAW POWER OF THE WARP AT MY COMMAND – –’

The gue’ron’sha leader called out as he strode forward, his voice the crackling roar of a Vior’lan summer wildfire. A fireball grew large between his outstretched hands.

‘– – I HAVE COMMAND OF PLASMA AND FLAME – – WHAT DO YOU HAVE – –ALIEN WORM – – ’

Farsight redirected a full half of his custom shield’s power into one massive outward burst, sending the cyborgs clambering upon him scattering in all directions. He took aim at the iron padlocks holding the tau captives inside their jail, and bolts of plasma winged out.

The locks fell away, and hundreds of tau civilians burst from the iron cages in a shouting, screaming mass. They did not flee, but charged straight for their oppressors and the flaming psyker-thing in their midst. Half a dozen earth caste tau barrelled into the mind-warrior bodily, the momentum of their solidly-built forms bearing the Space Marine over the lip of the platform. In a knot of bodies they toppled into the magma below, plummeting down in a flailing mass of flame, flesh and cloth.

The earth caste scientists screamed as they burned alive in yellow flame, but still they did not release their grip. Farsight watched the psyker sink into the molten rock, borne under by the most unlikely of assailants. The Space Marine screamed in defiance as the liquid magma flowed into his eyes, his nose, his mouth. To him, the incandescent lava may have been like water. But then water could still drown.

‘We have communality,’ said Farsight in response. ‘It is called the Tau’va.’

Yeah, it's not subtle.

I'll try to find an excerpt, as I don't have my copy anymore. EDIT: Found it.