Despite the absence of injured duo, Maanum hit two long range goals.
The winner of the group will beseeded and will play the second leg at home.
Stina Blackstenius hadn't scored in seven games.
Needing a victory to guarantee first place following their defeat at home to Lyon, the north London club immediately attacked the hosts and were rewarded by Maanum's classy free-kick opener.
Caitlyn Foord swept in Blackstenius' cross before she provided Maanum.
The Norway international set up Blackstenius to score from close range before the half as the team showed swagger.
Six minutes into the second half, Maanum hit her third from distance, and three minutes later, Blackstenius hit her second.
Foord capitalized on a missed back pass and made it seven, after Fabienne Humm pulled one back with a penalty.
Kim Little converted from the spot and Mana Iwabuchi did the same.
Lyon finished top of the table after a goalless draw with Juventus.
The draw for the Women's Super League will take place on February 10.
There is increased pressure on the forwards as they come to terms with the loss of Miedema.
In November, he had the same injury and was out for a long time.
Miedema's goals had earned her side a crucial win at Ajax and four points against Bianconeri on their way to reaching the knockout round, but Maanum's mastery in technique and imagination from set-pieces allayed any concerns that they might have had.
Maanum's approach from free-kicks is notchalance and he takes aim with devastating accuracy.
The attempt for her first goal, which floated over the wall and into the net, preceded an attempt that grazed the bar.
When she finished her classy hat-trick, there was no surprise.
If Maanum can maintain her goal scoring feats, a place in the semi-finals of the European competition for the first time in three years could be within reach for the north London club.
All Rights Reserved © 2024
