Cassius, a 15-year-old Noongar Yamatji boy, died in hospital 10 days after prosecutors say he was chased, knocked to the ground and "deliberately struck to the head" in Perth's eastern suburbs on October 13, 2022.
Aleesha Louise Gilmore, 23, her boyfriend Jack Steven James Brearley, 23, and his mates Brodie Lee Palmer, 29, and Mitchell Colin Forth, 26, are on trial in the West Australian Supreme Court charged with murdering Cassius.
A young man, who cannot be named for legal reasons, told the jury on Wednesday he and a group of fellow teens, including Cassius, caught a bus after school to a grassy open space to watch a fight.
He said a black four-wheel drive pulled up with some men and a woman yelling about smashed windows.
"One jumped out with a stick," he told prosecutor Ben Stanwix.
The witness, who was 16 at the time and now 18, said another man also got out of the vehicle and the armed man tried to hit one of his friends, but he dodged the attempted blow and then tried to punch his would-be assailant.
He said the man with the stick was skinny and tall and had white skin. He described the other man as short and stocky and said he was wearing a black T-shirt. He said the weapon was a plant stick and said it had a pointy end.
"The one with the stick he said to me: 'I'm coming for you, you little black c**t'," he said.
The men then allegedly chased most of the group of teens, including Cassius, into some nearby bushland with a creek running through it.
"They was running ... There was boys crossing the creek, boys on the other side," the witness said.
He said he spotted Cassius, who was a close friend of the witness, crossing the creek.
"He was in front of me, getting up on the other side of the creek," he said.
"I was scared. I thought I was going to die, so I kept running."
He said he heard someone shout out Cassius's name.
"I turned to him," he said.
"I seen the short man hitting Cassius, and then the other boy came across the creek and started hitting him too."
The witness said that after the attack, Cassius came out of the bushland area towards a TAFE college car park where the witness had run.
"He was hurt but he didn't say he was hurt," the witness said.
"His ear was chopped ... He said: 'I'm fine'."
The teen said the group of boys told Cassius to lie down and wait for an ambulance.
Mr Stanwix has previously said Gilmore had left Brearley, Forth and Palmer before they confronted a large group of teen students, who had gathered at a green open space after school.
He alleged Brearley chased Cassius down and struck him on the head at least twice. One blow split his left ear in half and another lacerated his forehead.
Mr Stanwix said Brearley "was filled with fury about his broken car windows", which happened a day earlier, and about threats communicated via social media that a group of "kids" could damage the home he shared with Gilmore.
Brearley allegedly later bragged about his "vigilante violence", saying: "He was just lying in the field and I was striking him with the trolley pole so hard so he learned his lesson."
Prosecutors say Forth, Palmer and Gilmore helped Brearley and knew his intent.
The smashed car windows were part of a series of escalating tit-for-tat incidents that started on October 9 when some of the accused allegedly "snatched two kids off the street" and unlawfully detained them, punching, kicking and stabbing one of them.
The incidents were triggered by a "love triangle" involving Gilmore's 14-year-old brother and another teen of similar age, and social media exchanges about the boys fighting.
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