Eighteen-year-old Amass was one of three changes Wednesday made as they aimed to improve on their total of just one point from their first five games.
But the youngster – making his debut after a deadline day loan move from Manchester United – had a nightmare start when Rob Dickie’s shot from a Mehmeti cutback deflected off him at close range and into the net.
Riis should have made it 2-0 just four minutes later when a Yann Valery mistake saw him through on goal, but Ethan Horvath did well to stop the onrushing forward as chants against Chansiri’s rang out.
Mehmeti was on hand to create City’s second when a poor clearance was cut out by Jason Knight and the Albania winger’s perfectly weighted ball into Riis saw the striker calmly stroke the ball through Horvath’s legs.
The impressive Mehmeti got a goal for himself as he drifted off his marker at the back post, beautifully controlled Scott Twine’s cross and arrowed in a shot in from the corner of the six-yard box to make it 3-0.
He should have had two more in the next seven minutes. First Horvath got down well to palm a fierce effort away and then, having rounded the keeper, Mehmeti found the side netting from a difficult angle.
Ike Ugbo went closest for the Owls when he put the ball wide at the far post from Max Lowe’s cross as the hosts struggled to match City’s passing and incisiveness.
After the break Mehmeti came inches away from a second when his left-footed first-time effort thumped the post after good work by Ross McCrorie, before Wednesday’s woes were increased when Amass limped off a couple of minutes later.
Jamal Lowe had a rare effort for the hosts with 16 minutes left as he broke down the right before blazing high over the Robins’ bar while Mehmeti curled an effort just wide moments later.