The pair began to trade blows when Skeen allegedly intervened, punching Mr Tipene's friend in the back of the head.
Ms Johnston said Mr Tipene responded by punching Skeen, who ended up on the ground.
While there, the Crown says the defendant grabbed a broken bottle.
"Vincent Skeen stood back up, clearly enraged, and was not going to leave matters at that," Ms Johnston said.
She told the jury the teen rained down blows on the "dedicated and promising" athlete in a downward "hammering" motion.
Nadene Togiavalu, 19, told the court how she drove her cousin, Mr Tipene, from west Auckland to the party because he had voiced his concern about a friend's safety.
When they arrived in Grey Lynn, she said a circle formed around two boys who were fighting.
Ms Togiavalu said Mr Tipene's friend was "getting a hiding" after being attacked by several other boys.
As she tried to persuade her cousin to walk away from the melee, an altercation developed between him and one of those who had set upon his friend.
Once the scuffle was over, Ms Togiavalu - who struggled through tears - said another boy then entered the fray.
"He picked up the bottle and smashed it, then he stabbed it in Luke's throat," she said.
She told the jury the attack came from the side and Mr Tipene was left holding his neck, trying to tell her he was going to be alright.But he soon collapsed beside the road.
"He was trying to get the word "ambulance" out but he couldn't talk because there was blood all over his face . . . it was like squirting out."
It is alleged Skeen hit Mr Tipene with the sharp bottle neck at least seven times.
Ms Johnston said a pathologist would give evidence of 13 sharp-force injuries to the victim's body.
The fatal blow was nearly 12cm deep and pierced Mr Tipene's jugular vein on the left side of his neck.
Bleeding profusely, an ambulance rushed him to Auckland City Hospital, but he died at 4.45am.
Skeen's lawyer Lorraine Smith accepted her client was responsible for killing the teenager but said he was not guilty of murder.
"At the time Vincent swung out holding a broken bottle, he had no appreciation his actions could or would cause another boy to die," she said.
Mrs Smith said Mr Tipene was "bigger and stronger" than Skeen and had punched him to the ground twice before the fatal blow.
She conceded the defendant's actions were "very foolish" but told the jury they would have to consider whether he intended to connect with the victim.
"If Vincent is guilty of anything, he's guilty of manslaughter," Mrs Smith said.
One of the first police officers on the scene described the aftermath of the melee to the court today.
Constable Ray Fa'aofo said he had to calm down a couple of angry young men before turning his attention to Mr Tipene, who was lying on the pavement while someone applied pressure to his neck with a jacket.
Mr Fa'aofo said he immediately realised the teen was in trouble.
"There were pools of blood underneath him. I knew it was quite serious because he had lost a lot of blood," he said.
When he took over applying pressure to the wound, he said Mr Tipene started coughing up blood as his eyes rolled back.
The trial, before Justice Mark Woolford and a jury of six men and six women, is expected to last up to three weeks.
