Checkpoint

Let’s pull all these fragments back together and see what we have. Here’s the original snippet:

He edged a little closer to see what they were attacking.

Then it roared and reared up on its hind legs, scattering the men –

– and Lucky saw horns, great curving horns, pointing up at the sky –

– and his stomach turned to liquid. His blood ran cold.

Because it was an Alien. The first he’d ever seen in real life.

It was a young male, and he was big. He was way bigger than Lucky. He was as big as the biggest man in the crowd. He was muscular, too: a solid wall of muscle, hulking and roaring under the lights, like a nightmare come to life.

He was dressed in a strange coat that looked like it was made of liquid metal, billowing behind him like wings. His flaming eyes were covered by mirrorshades, but Lucky could see cloven hooves protruding from the bottom of his coat: massive black hooves that could crush a Human head.

Phoenix

Here are the examples we've been building:

He adjusted his position to see a little better, all concern about being caught as a stowaway forgotten.

The creature roared, unfurled its tentacles and tried to swat the shooting soldiers –

– and Biff saw teeth, rows and rows of teeth, going down its throat as far as he could see –

– and his throat went dry, his eyes nearly popped out of his head.

The creature was the most horrifying thing he’d ever seen.

It seemed to have no bones, something Biff had never seen in a creature that large. And large it was; it would have easily filled the cafeteria he worked at, and then some. It also had at least twenty eyes over what he guessed was its head: twisting and turning, angrily locating each of the soldiers surrounding it, like some demonic, mutant octopus.

Its long tentacles writhed, hook-like barbs on the tip of each one obviously designed to catch its prey. The creature roared again, louder this time, spit escaping from its mouth and spraying over the soldiers in its path. Biff could see the soldiers writhe on its impact, as if the creature’s spit was poison, or acid.

She hit the brakes and looked through a scope.

It rose from the desert floor, sand streaming off its back —

— and Vijay saw black legs, shining black pincers, glistening in the sun —

— and her heart beat faster. Her skin prickled.

She had woken a saffak. A desert hunter.

It resembled a giant black crab. Huge, taller than a house, so tall Vijay could have driven a bombtruck between its legs and parked in the shadow of its body. But it was also fast. Four spiny black legs scuttling fast across the sand, and two barbed pincers stabbing the air in front like a harvester of death.

The saffak was armoured with thick black shell, all bumpy and ridged, shining glossy in the burning sun. The gloss came from sweat that streamed out of tiny holes in the shell, and as the sweat dried it turned to sulfur, leaving streaks of yellow powder and the smell of rotten eggs.

She peeped over a row of coral to catch a glimpse of it.

Was it one human? Ten? Attached to a machine that propelled it (them?) as fast as any fish –

– and Jola saw spears, long metallic spears, aimed at a school of parrotfish just going about their lives –

– and her heart began to pound. Her body stiffened in fear.

This was the Enemy. Far more terrifying than the briefings and training exercises implied.

As each human broke away from the machine, Jola counted four of them. Even individually, they were twice her size. They wore the black skins and breathing devices she learned about in training, too: obviously designed for them to invade our world, despite not belonging here, like spoilt children wanting what they couldn’t have.

They had lights coming from their heads. Whether this was artificially generated or a natural feature of humans, Jola didn’t know. Their feet were long and webbed like amphibs, but this looked constructed, as they seemed to be made of the same substance as the black skin they wore. They each tapped on arm bands that lit up on contact. Did they have nav devices too, she wondered? But Jola’s thoughts were quickly dispelled when the spears the humans carried began to shoot from their hands.

And below is your version, joined together. You'll need to delete the line breaks to create a continuous paragraph.

Is there anything you want to edit? This is your last chance to make improvements before we conclude the lesson!

Make sure you've:

Started with a key feature that identifies them as being alien or different.
Explained how the alien behaves, and how the protagonist feels about them.
Expanded on the alien's physical features.
"Coloured in" the rest of the description by highlighting further details.

Delete excess paragraph breaks and polish your scene.

Science fantasy worldbuilding in Phoenix 4: Close encounters