The now 18-year-old, who cannot be named for legal reasons, faced the Supreme Court of Victoria on Monday after previously pleading guilty to the manslaughter of 17-year-old Jason Langhans.
Crown prosecutor Jane Warren told the court Mr Langhans, an apprentice plumber and keen fisherman, was acting as a peacemaker at the gatecrashed 16th birthday party in Tooradin when he was struck in the head in March 21.
He died three weeks later in hospital.
His attacker was 17 at the time.
Family and friends described Mr Langhans as a "kind and caring" young man who had his life taken away in one fell swoop.
"Who takes a screwdriver to a girl's 16th birthday party?" his grandparents Cheryl and Peter said in their victim impact statement.
"We wish (his attacker) nothing but pain and suffering for the rest of his life. May he rot in hell."
His father Paul Langhans wept while telling the court how he begged for the doctors to "save my boy".
"I watched him take his first breath when he entered the world and watched him take his last," Mr Langhans said.
"My heart is shattered."
The court heard the teen, only referred to as JN, had been diagnosed with post traumatic stress disorder and paranoid schizophrenia. He had consumed half a bottle of vodka and smoked cannabis before attending the party uninvited, armed with a screwdriver.
He later drove that screwdriver almost 8cm into his victim's brain.
Psychiatrist Rajan Darjee told the court the teen's intoxication, coupled with his PTSD and paranoid schizophrenia was not a good mix.
The court also heard of the teen's early life in Afghanistan, his arrival at Christmas Island detention centre and his poor experience at school once he moved to the mainland.
The teen was remanded in custody to be sentenced by Judge Elizabeth Hollingworth on August 24.
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