Melbourne will host Geelong in a home elimination final after steamrolling GWS by 45 points at the MCG to secure fifth spot on the AFL ladder.
The Demons were reduced to two men on the bench for Sunday afternoon's second half but ran riot in the third quarter, piling on five unanswered goals on the way to a 15.12 (102) to 8.9 (57) triumph in front of a 37,285-strong crowd.
Melbourne will take on the Cats at the MCG in the first week of the finals while GWS, who finish the season in seventh place, face a sudden-death clash with cross-town rivals Sydney.
The Dees head into their first finals campaign since 2006 full of confidence after backing up last week's win over West Coast with an imperious performance against the Giants.
Brownlow Medal contender Max Gawn continued his superb season with 37 hitouts, 18 disposals and a goal, Christian Petracca and Tom McDonald each booted three majors, and Angus Brayshaw (32 touches) was prolific in the midfield.
"We've been working really hard to play our best footy at this time of year," Demons coach Simon Goodwin said.
"Our contest was strong, (as was) our clearance work and I thought we defended incredibly well, which is something we've really worked at for the majority of the year."
Prime mover Stephen Coniglio (32 disposals, two goals) was influential for GWS but Josh Kelly (19 touches) became the latest star to struggle under a hard tag by James Harmes.
Giants ruckman Rory Lobb played no part in the fourth quarter after experiencing tightness in his back.
Melbourne lost Dean Kent to a shoulder injury in the opening minutes and Dom Tyson (broken arm) joined him in the rooms late in the second term.
Tyson will undergo surgery on Sunday evening and the Dees remain hopeful he will play a part in their finals campaign.
The injuries mattered little, with the Giants looking out of their depth in just their second appearance at the MCG this season.
Melbourne led by just 13 points after a tight first half but their dominance at the contests always looked likely to pay off eventually.
The Giants' woes were exemplified late in the third quarter when Jeremy Cameron looked to handpass to Zac Langdon in front of an open goal but failed to hit the target, prompting jeers from the Melbourne faithful.
"We stopped after halftime ... that's disappointing," Giants coach Leon Cameron said.
"We've put ourselves in a position to play finals footy but we've got 13 days to rectify it."
GWS were missing at least eight members of their best 22 but star forward Toby Greene, veterans Brett Deledio and Matt de Boer, and key defenders Aidan Corr and Sam Taylor are a chance to return for week one of the finals.
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