That pummelling left the teen with chipped teeth, an eye that was swollen shut, and lacerations to the top of his head, Judge Cathcart said.
In a victim statement, the boy said he was shocked by the sight of his blood everywhere and still had no idea why he was targeted.
He was fearful of making a statement.
Raroa had asked him if he was from Frasertown - trying to connect the boy to the rival Black Power gang, which had a stronghold there, Judge Cathcart said. But the boy had answered no.
Raroa admitted the offence to police but offered no explanation other than to say he was drunk.
Counsel Alistair Clarke sought home detention, saying that, while Raroa was not shrinking from the fact this was gang-infused drunken thuggery, he had considerable potential to do better in life.
A pre-sentence report writer had taken an almost stereotypical view of Raroas gang membership, but it was something he had virtually inherited rather than actively aspired to achieve.
Balancing out that negative, Raroa had numerous personal qualities.
Those who knew him described him as intelligent, hard-working, and respectful, with real leadership prospects.
He was more than capable of rehabilitation and had no history of breaching any court-imposed sanction.
But Judge Cathcart said only imprisonment would do.
This was gang intimidation and violence at its worst, typical of crime in the Wairoa area.
It also had Raroas personal stamp.
The court not only needed to deter Raroa but others - particularly other gang members who preyed on innocent people in that community.
The victim was of crucial importance and his interests must also be protected, the judge said. While Raroa was described as having immense potential, that did not mean the court could ignore what he did to that boy.
The judge agreed with Mr Clarke that the sentence starting point should be two-and-a-half years imprisonment.
Deductions were applied for Raroas guilty plea and willingness to make a payment for emotional harm.
In addition to the prison term, there would be standard and special release conditions for six months.
If Raroa was serious about rehabilitating himself, that would be his opportunity, the judge said. He also ordered Raroa to pay $200 to the boy for emotional harm.
No reparation had been sought for dental or medical expenses.
There was no uplift for Raroas previous offending, although the judge noted he had previously received a sentence of three-and-a-half years imprisonment on a charge of participating in a criminal group.
While cleared of any actual violence on that occasion, the charge by its nature showed Raroa had been associating with a gang for the specific purpose of carrying out violence, the judge said.