Ms Anderson had moved in with her partner, whose second cousin, Mr Forsythe, lived next door.
Crown prosecutor Dan Boyle said Forsythe was invited to the gathering, where all but one person were drinking from about 4.30pm.
He said drugs were also consumed during the afternoon and evening.
The court was told Forsythe became increasingly aggressive throughout the party and challenged Mr Dumper to a fight more than once.
Mr Boyle said Forsythe then later went to his house next door and returned with a shotgun.
He said Forsythe did not say anything before shooting Mr Dumper.
"He deliberately shot Brent Dumper in the back in an offence that can only be described as cowardly,'' Mr Boyle said.
A second man, Michael McMillan, was shot at but managed to push the shotgun away, though pellets put holes in his clothing and grazed his stomach, the court was told.
Forsythe told the court he had returned home to get his gun after feeling "a bit humiliated''.
He said Mr Dumper unexpectedly grabbed the gun and he accidentally applied pressure to the trigger which activated.
Forsythe said he did not know how the shot got Mr Dumper in the back after the "struggle''.
"He's obviously turned ... or we pushed him,'' he said.
"There was no time to analyse what happened. It was too fast. I was in a panic so I tightened my grip (on the trigger).''
Mr Dumper died soon after with 160 pellets in his body.
Witnesses told the court Forsythe simply shot mr Dumper in the back as soon as he returned with the gun.
Forsythe told the court he kept the gun loaded in a wardrobe because he had a home invasion about two years before and also feared trouble from his partner's family who had been causing problems.
The judge will sum up tomorrow before the jury retires to consider its verdict.
