The 18.12 (120) to 9.13 (67) defeat was Essendon's third-straight loss amid strong speculation the club is only a day or two away from learning of severe AFL punishments over the supplements scandal.
Essendon couldn't match it with the Eagles in an error-riddled game and dropped away horribly in the last quarter.
The loss sees the Bombers drop to seventh on the ladder.
In a further blow, Dustin Fletcher was reported for a high bump on Jamie Cripps in the final term that left the Eagle dazed.
The hit may see the veteran wait a little longer to play his club-record 379th game after equalling Simon Madden's games mark on Sunday.
Important small forward Jason Winderlich also suffered a suspected hamstring injury.
By contrast, this was West Coast's best win this season.
The Eagles started the round 11th, three games outside the top eight and now sit 10th and equal on wins with ninth-placed Carlton.
Eagles forward Mark LeCras returned from having his ribs crunched in the second term to star with five second-half goals.
West Coast midfielder Matt Priddis also starred, picking up 28 disposals.
The tone was set in the first three minutes when Andrew Gaff was paid a free kick immediately after the opening bounce and goaled after receiving a 50m penalty.
Josh Hill was then awarded a goal from a contentious video review to give the Eagles the early break.
Essendon hit back immediately to lead by seven points, but West Coast dominated the end of the first term and should have been further ahead than three points.
The two teams went goal-for-goal in the second term as Essendon and West Coast players constantly turned the ball over.
After Essendon small forward Alwyn Davey handballed through a point for his team in the third quarter - he was trying to find Nick Kommer and missed him - West Coast kicked three goals to open up the game.
The Bombers briefly fought back, but the Eagles closed out the quarter with another three goals to lead by 21 points at the last change.
Essendon's Brendon Goddard kicked the first goal of the final term to briefly keep it interesting, but from there the Eagles ran riot.
Michael Hibberd and Dyson Heppell were best for Essendon.
ESSENDON 3.3 6.7 8.13 9.13 (67)
WEST COAST 3.6 6.9 12.10 18.12 (120)
GOALS
Essendon: Goddard 3, Winderlich 2, Kommer, Bellchambers, Hooker, Baguley
West Coast: LeCras 5, Hill 2, Wellingham 2, Darling 2, Dalziell 2, Schofield, Priddis, Kennedy, Gaff, Cripps
BEST
Essendon: Goddard, Hibberd, Watson, Carlisle, Stanton, Heppell
West Coast: Priddis, LeCras, Darling, Masten, Cox, Dalziell
INJURIES
Essendon: Winderlich (hamstring)
West Coast: Wilson (head)
SUBSTITUTES
Essendon: Ben Howlett replaced Jason Winderlich during the second quarter
West Coast: Brad Sheppard replaced Blayne Wilson at three-quarter-time
Reports: Dustin Fletcher reported for rough conduct against Jamie Cripps
Umpires: Leppard, Hosking, Meredith
Official crowd: 32,595 at Etihad Stadium
