A friend and I used to play this a lot when we were roommates. We developed a strategy using suped-up Kelpies.
We used to pick in the AI, playing a 2v1 against the easiest bot in order to grind out new cards for our decks. Dunno if he ever beat the story, but I couldn't. There was a particular level against a water-themed opponent that I couldn't beat. I didn't want to keep trying because the framerate would be choppy in that level and it'd give me a headache. Shame, because you earned some pretty good cards through the story after that level. There are some other games in the series, like the one on the 3ds. I owned it, but my 3ds broke shortly after I got it and I never got to play it much. I'd love if they were preserved on GoG, and it'd be even more amazing if the series got a new entry.