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.
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.
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.
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.
Seoul will hope Popovic's men decide to attack at home, but that could mean Western Sydney dominate possession and dictate the tempo.
