The Sydney coach had told the fans mid-week to expect a one-sided contest, but the Bombers had to be content with a 13.12 (90) to 10.11 (71) victory in the 15-a-side contest.
Ted Richards was the stand-out for the Swans up forward against his former club, showing the way with 21 touches, three goals and solid marking in an otherwise scrappy affair.
Lewis Roberts-Thomson (18 possessions) played strongly on Scott Lucas, Luke Vogels held Matthew Lloyd to just two goals, while Jude Bolton, David Spriggs and Nick Malceski performed strongly in the middle.
Meanwhile, Kepler Bradley (18 disposals), Jason Winderlich (15), Andrew Welsh (20 and) and Scott Lucas (two goals) were the stand-outs for the Bombers, who ended up resting former skipper James Hird.
Ablett put the home side in front with the first goal before Lloyd snagged a major for the Bombers.
Grundy then showed why he was promoted from Sydney's rookie list with a solid mark and goal from 40 metres to give supporters hope of a competitive game.
While Spriggs and Scott Camporeale welcomed each other with a few shoves and Joel Reynolds made sure Bolton knew he was in a competitive contest, the visitors piled on the last three goals to take an 11-point lead into the first change.
Spriggs reduced the deficit before the Bombers once again quashed any hopes of a close contest with four unanswered goals, including a nice dash through the middle and snap from Nathan Lovett-Murray.
Richards posted his first goal to get the crowd excited and although Lucas nailed his second, the Swans posted the final two goals to trail by three goals at the main break.
An uninspiring third term saw the Swans out-score the Bombers two goals to one, with Richards adding his third, to peg back the deficit to 13 points at the last change.
Jobe Watson booted his second soon after the centre bounce before Sydney draftee Ryan Brabazon ensured the Bombers wouldn't have the game all their own way to the applause of the crowd.
Chris Heffernan and Ricky Dyson sealed the victory for the Bombers but not before Swans rookie Paul Currie slotted through a beautiful curler from the boundary line 20 metres out.
Essendon coach Kevin Sheedy said the main positive was no injuries but he was not completely pleased with the performance.
"It was a game that was up and down, we felt we should have got more scores on the board and with the ability out there we should have," Sheedy said.
"We haven't done a lot of goal-kicking at the moment but we will see if we can zone in on that next week.
"But full marks to Sydney, they had a lot of young players out there trying their butts off and it kept the game interesting all night."
Roos was surprised by the final margin and called it the Swans' best team effort at the ground given the imbalance of the two sides.
"The score was obviously a really pleasant surprise for us, and we are all really excited the way the kids played," Roos said.
"We played some great footy and the kids attacked the football and even the improvement from LA (last month) to now is just dramatic."
SYDNEY: 2.5, 6.5, 8.8, 10.11 (71)
ESSENDON: 4.4, 9.5, 10.9, 13.11 (89)
GOALS – Sydney: Richards 3, Currie 2, Ablett, Grundy, Spriggs, Thornton, Brabazon
Essendon: Lucas 2, Lloyd 2, Watson 2, Monfries, Dempsey, Welsh, Lovett-Murray, Ryder, Watson, Heffernan
BEST – Sydney: Richards, Roberts-Thomson, Spriggs, Bolton, Malceski, Vogels
Essendon: Bradley, Winderlich, Lucas, Welsh, Monfries
INJURIES – Sydney: Nil
Essendon: Nil
REPORTS - Nil
UMPIRES - Troy Pennell, Matthew Head, Mathew James
CROWD - 8461 at North Sydney Oval, Sydney
