So, Winter vs. Spring is a fun way to pass the time. Problem is I'm not sure how easy it is to louse (I never lost even after spending half the stage attacking myself on accident). Also I'm kinda tired so this won't be well organized. Also this does contain a strategy to win the game so if you don't want that, don't read it.
Once you understand the key to playing the game it becomes easy to win. That key is that this game loves bottle necks more then Suika and King Leonidas combined. Trap your enemy (especially Lily, because dispite everything, she is the strongest) on one side of a key node, preventing them from taking more then half of the area, take most of the area and then attack repeatedly. The problem is that you are are being lead by Cirno who displays her complete inability to place units onto a battle field. Let me demonstrate with screenshots from Stage 6.
Here is your first move of Stage 6 and
here is your second. God dammit.
So, especially later in the game, getting your guys out there becomes more of a problem. Your enemy also has a head start with more and better units.
The problem is that you still aren't screwed. The AI is kinda sad. Ok, so once you find and start blocking the bottlenecks. It may be understandable that your opponent doesn't just make a break for it to stop you (that kind of AI might be hard to construct). What really annoys me about the AI is that it makes attacks that it knows it will louse.
Suggestions:
Units blocking each other would be awesome if you were the one placing the units at the start of the game, but you don't making the inability to place two units on one node very annoying. This might also be really easy to implement.
Better AI. I don't want to say it would be easy to check if the the game is still winnable but I think it might be easy to check if the game is still winnable. That's a start. Maybe you can also do a check for key points on the map... I dunno. If you make you're AI much smarter without allowing for unit walking or unit placement, the game will quickly become un-winnable.