Carlos Tevez was the star of the show for City with a hat-trick and Aleksandar Kolarov and David Silva also scored.
Flitcroft's Championship relegation scrappers were outplayed from the first whistle to last and in the end were lucky to get away with not conceding more.
"If they are the second best team in the country, I wouldn't like to have played Manchester United," Flitcroft said.
"They were ruthless. They battered us to a pulp at half-time and then come out in the second half and tried to drill us into the floor.
"That left a massive impression on me. It is what champions are about.
"They wanted to floor us today. They wanted to get seven, eight, nine, 10 goals. At times it was a real slow death.
"The only thing is we won't come up against a force that is Manchester City again. You don't face that kind of power in the Championship."
Flitcroft singled out Tevez for particular praise, saying: "It is his power. It is his explosion and movement. It is incredible.
"He is a ghost. We talked about getting close to him in the box but the players said he just pulls off defenders.
"Every shot is on target. He is brilliant."