Flitcroft also revealed that he has taken inspiration from Bradford City's march to the final of the League Cup.
"We spent a lot of time and dedication preparing for this tie," BBC Sport quotes Flitcroft as saying. "I think teams have underestimated MK Dons in the past and that's something we didn't want to do. Our first-half performance was a masterclass.
"We're in the last eight of a competition I absolutely love. I have been really inspired by how Bradford have done their business and their cup run been very inspirational for this group. I'm trying to make new heroes at the club."
Barnsley join Millwall and Blackburn in the quarter-finals, who also won on Saturday.