She punches it in the face with a diamond spike. This predictably does little to save the rest of the people there.
And Mountain is just about tapped out now. She flies somewhere (relatively) safe, one of the locations she'd been evacuating to with all the red tents, and pretty much collapses, still in her armor.
When she wakes up, perhaps two hours later, she can't detect it. She finds someone not busy being badly injured or helping people who are, makes a stone caricature of the monster and points it in various directions. Where did it go?
She nods and flies that way, fixing a few roads that have visible amounts of stuck traffic but otherwise saving her strength. She follows the path of destruction until she finds the monster or the path ends, meaning it was killed or gave up.
as it heads back the way she's coming. Through the rain-caused darkness she can see quite a lot of destruction there as well, and the creature doesn't stop to greet her. It stops for a second to topple a building, and she'll be able to notice it's visibly injured, but that doesn't seem to be affecting it in the slightest.
She untopples the building. Then starts trying to slow it down again. She's visibly slower and less powerful than when she fought it before, but she's still damn well going to try. Who's doing the most damage to it, so she can prioritize protecting them?
There are apparently a handful of people who seem to be capable of dishing out the hurt: one human male in a green bodysuit with metal armor who's throwing lances made of energy at it, one in a blue-and-white bodysuit who can shoot lasers with various effects, and one female who can seriously pack a punch. The one in the bodysuit teleports himself and the other two whenever the monster gets too far out of range, and they don't seem to be needing much protecting.
She can't do much more big, expensive manipulation like strengthening the island's rock underlayers or making plateaus to evacuate to, but growing hills around the monster's legs is nowhere near the same scale.
She's not afraid of getting smashed, whatever sent her here instead of to her own temple besides. But seriously, what the hell is that thing? If this was her home the entire continent would have crumbled into the abyss by now.
She keeps doing her best to immobilize it, getting angrier and angrier at the pointlessness of its rampage by the minute.
And there's... someone there? Hard to make out from a distance, but they look... taller than a regular human, if still shorter than the beast. And the silhouette doesn't look... quite... human...
Is that person one of the fair folk? It looks a bit like an Ignivore, what with all the fire. Eh, no time to think about it. Monster's head, meet that diamond spike again. But it manages to grab and crush and throw her after that blow. Ow.
One tidal wave hits the building the fiery stranger was on, and they disappear for a moment before emerging again and charging for the monster.
But this... Giant not-tile... Is more and more damaged by the minute. The monster is shaking it to pieces. Well, she knows stones. Earthquakes, go away you are not welcome here.
The human with the lasers and the human that throws giant balls of light stop doing that when the... other thing... starts hitting the monsters. Okay, now there are two monsters and nicknames may start becoming necessary.
The big one throws the little one away, though, and the two humans start shooting it again. Oop, new tidal wave!
She's not really aware of anything but the slowly, slowly failing structure of this place's ground. She pours everything she can manage into keeping it stable, barely noticing that she's making a miniature mountain in the process, with ridges radiating off for miles in all directions. She can't afford to try to keep what's left of this city in one piece if she wants to try and save the rest of the island.
I am the mountain. The sea will not break me, and it WILL NOT break this island.
At some point she falls unconscious, mostly embedded in her shiny new mountain.
She digs herself out of the mountain. She drinks some water pooled on the mountainside, then flies down to investigate the devastation.