The Eagles took a surprise lead early in the second quarter of Sunday's western derby, but Fremantle steadied to seal the 14.13 (97) to 5.11 (41) win in front of 54,232 fans.
It was Fremantle's ninth win from the past 10 derbies, but West Coast still hold a 33-29 lead overall.
The Dockers' fifth win on the trot kept them in second spot on the ladder with a 5-1 record, behind Sydney on percentage only.
All-Australian defender Jordan Clark won the Glendinning-Allan medal as best afield for his 27 disposals, one goal, 622m gained and 11 score involvements.
But it was Amiss who was 'the man' in attack, finishing with a club-record nine marks inside 50m, edging the previous high mark of eight held by Dockers great Matthew Pavlich.
Amiss had kicked four goals on nine previous occasions, but he finally cracked through that barrier early in the final quarter when he nailed a 50m set shot.
The 22-year-old also notched 13 disposals, one goal assist and 5.4 as West Coast struggled to cover all bases in a tall Fremantle forward line containing Amiss, Patrick Voss (three goals), Josh Treacy and Luke Jackson.
Earlier this season, calls were growing for Amiss to be dropped after he kicked just one goal in the opening two rounds.
But 13 majors in the past four matches - including a clutch long-range late goal against Collingwood - have well and truly silenced the doubters.
"Throughout all the challenges he's had, he's always been positive," Longmuir said.
"He's always kept working on his game. He's got his feedback internally, and it's good to see him getting some of the rewards.
"Jye's actually stood up for us a lot under pressure, whether it be kicking goals when we need in the last quarter, which is the obvious part.
"But also throughout last year, there were a lot of times when we just needed a contest from him on the wing, when the opposition were coming at us, or he was two-v-one against, and he was just able to get it done."
Andrew Brayshaw sustained an early knock and only finished with 15 disposals and three clearances, while former Collingwood ruckman Mason Cox tallied four disposals and 13 hit-outs in his Dockers debut.
Jake Waterman was lively for West Coast with 11 marks but was wasteful in front of goal with 1.4.
Harley Reid (20 disposals, three clearances) battled hard, while Reuben Ginbey performed an excellent job in keeping Josh Treacy to one goal.
West Coast kicked two goals early in the second term to threaten Fremantle, but the Dockers soon put last year's wooden spooners in their place.
Amiss kicked two goals and Jackson added another, before Harley Reid gave away an unlucky 50m penalty for tackling Clark to the ground thinking the kick hadn't travelled 15m.
Voss taunted Reid after Clark kicked the goal, with Fremantle heading to halftime with a 22-point lead and all the momentum.
West Coast won the inside 50m count 51-50 and contested possessions 124-120 for the match, but their scatter-gun entries meant they couldn't come close to kicking a winning score.
"Funnily enough, I know the scoreboard doesn't reflect it, but I was reasonably pleased with the way we played today," Eagles coach Andrew McQualter said.
"We were able to beat Fremantle in entries, beat them in contest, big focus areas for us going in. But we were just clearly really inefficient going forward."
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