So far the game works well with just one unit on both sides. Up the ante by adding multiple units. I suggest the following:
Add multiple enemy units first, with one player unit. Make them weaker units to test winnability. (Look for overlapping with each other around the player, a unit moving more than allowed taking another unit's turn, etc)
Then add multiple player units to one enemy unit. (test which one the enemy goes after, if the pathfinding gets confused, and player ability to switch between them and choose at any time during their turn)
Then open the bag of worms that is multiple units for each side.
So far the game works well with just one unit on both sides. Up the ante by adding multiple units. I suggest the following: