I think there are a bunch of ways you could do the score. Why don't you try drawing out a paper board (Unless the board and enough enemies are already in that you can actually play it to test different mechanics) and try out a few different rules? Like try it with all enemies giving the same score, one where enemies give different scores, one where enemies can damage the player, etc and see which one is the most fun. At least that's what I've done before to try and test out a few turn based games I want to make some day lol.
Hmm... currently I've implemented a simple score system which increments by one every turn and also for every enemy killed. Could change that, but now I'm thinking of ways the very board could affect gameplay. Maybe the crossbow could have pickup-bolts? Or certain modifiers could be applied(like Double Action Points and 1.5xScore, but movement restricted to that of a Chess Knight)