The Wanderers battled to a 0-0 draw in the first leg in Seoul, putting the A-League side in a good position ahead of Wednesday's clash.
Tony Popovic's side are looking to become just the second Australian team to reach the decider of Asia's premier club competition.
In contrast, South Korea have had a representative in the final in every year since Adelaide United were beaten by Japan's Gamba Osaka in 2008.
Seoul are two-time runners-up, including last year, but have struggled to find their best form in the K. League.
Choi Yong-soo's men are unbeaten in 13 games in all competitions, yet have battled for goals this season – Seoul have just 31 in 28 league matches.
With his team fifth in the league, Choi is under some pressure – but that could change if he can help deliver the club's first Champions League success.
Making things more difficult for Seoul in their search of goals is the absence of Yun Il-lok.
Yun has scored eight goals in all competitions this year, but the 22-year-old is away with South Korea's under-23s team at the Asian Games.
Seoul are without a Champions League goal since their round-of-16 win over Kawasaki Frontale but they still managed to see off the Pohang Steelers in the quarter-finals.
In contrast, the Wanderers regain their best striker.
Tomi Juric, who scored in each leg of the Wanderers' quarter-final win over Guangzhou Evergrande, returns from suspension.
The five-time Socceroo's return is a huge boost for a Western Sydney side whose biggest strength is their defence.
Juric adds much-needed firepower and the Wanderers may need it if they concede early in front of their vocal home crowd.
Seoul will hope Popovic's men decide to attack at home, but that could mean Western Sydney dominate possession and dictate the tempo.
Both teams may only need one goal to decide the tie, making the opener of the utmost importance.
