Mohun Bagan had already qualified for the pre quarter-finals after emerging as leaders of Group G. They finished with 11 points from six games, with three wins and one defeat
South China are the second team to qualify from the group with 9 points from six outings.
Strikes from Law Hiu Chung (19), Ryan Griffiths (24) and Mahama Awal (33) sealed the deal for the visitors who had lost the first leg 0-4.
Mohun Bagan rested several of their regulars. But started off on a strong note and dominated early possession.
Subhash Singh could have helped them get a lead but his strong shot in the 14th minute fizzed past the post.
The visitors then knit an attack of their own and Law pounced upon the opportunity which was presented to him.
It was Awal who tore down the right beating his marker and sent a low cross in. All Law had to do was get his foot to it and he obliged.
Barely had Mohun Bagan recovered when Griffiths strike jolted them again. This was a tap in too as the hosts’ defence was caught napping.
The home team seemed to lose the plot after the double shock and South China took advantage of it again.
Awal got lucky in the 33rd minute as his strike took a deflection and looped over the Mohun Bagan keeper Arnab Das Sharma. The goal gave South China a 3-0 lead at halftime.
Mohun Bagan looked brighter after the restart, but whenever they pushed upfront, South China tried hitting them on the counter.