Two or three police cars arrived soon after to the house, which is near Burundi Ave.
"They were armed but they were really good about it, the way they handled it. I was impressed. They didn't race down there like idiots, waving their guns around.
"They made sure the other neighbours were inside and went down there nice and quiet and sorted it out."
The victim - estimated to be in his early 30s - was taken to hospital in an ambulance.
Police confirmed a man had been assaulted and taken to hospital with a serious injury. "There's been an allegation that a firearm was presented but that's not confirmed at this stage," a spokeswoman said.
Police had not arrested anyone but were making enquiries.
Armstrong did not know the people who lived at the property behind his house, saying they were new to the area.
But he said his patch of Clendon had been "brilliant" until now. The 49-year-old and his wife moved in 21 years ago and have raised two children there.
"We've had no problems. Me and my other neighbour say the idiots move in and...then they move out. We love it."
But he admitted the incident "makes you not like the neighbourhood, a little bit".