Chain combos aren't quite as they sound. You don't really need to do many combos at all. This counter is merely driven by enemies defeated rather than number of hits in a single string of moves or something like a traditional combo system. You are given somewhere around 5 seconds to defeat enemies or break crates before the counter vanishes, merely hitting the enemies does not preserve the chain.
The earliest stage I found this possible on was the West Coast Canal Industrial Complex in which Baek is the opposing boss. Before reaching Baek, I had reached a chain of approximately 54 while keeping a lookout for both enemies and boxes alike. Baek has a few goons with him, so if you haven't quite reached the mark you'll be able to keep your streak alive as this fight doesn't interrupt the counter. I'd suggest targeting his lackeys before him however, as they will be K.O'd faster and keep your chain alive rather than trying to kill him and failing because you weren't able to K.O him fast enough. Again, this was the earliest and simplest stage I could find however I'm sure there are many stages past this one in which this is possible.
EDIT: This trophy is practically a feebie on the stage G corporation, Millennium Tower Heliport. It's the stage in which you face a mob prior to fighting Kazuya, and you'll easily rack up over 60 enemies chained if your gear is decent enough.