The AI doesn't actually look for combos, and due to the complexity of that it's not something that's planned, either. The VS AI for Tidalis is fairly limited, as advertised, as it simply wasn't a focus for this game. Sorry!
But that didn't let us stop putting in VS modes that the humans would be good at and enjoy -- and we let the AI play in all of the ones that it technically can play in, even if it's not going to be very good at all of them (it's pretty good at the VS Garbage, for instance.
In short: the scope had to end somewhere, but we didn't want that to affect the non-AI-related VS offerings.