I'm surprised you managed to ambush the AI with concealed units. For some reason the enemy loves to head directly for my hidden units. They do not pass Go. They do not collect $200. They ignore my visible units and plow straight into the hidden ones. I play on VH/M, so maybe the AI units have ESP when the Strategic map is set to Very Hard (which would probably indicate a bug since, if the game's going to cheat like that, it should do so only on Very Hard Battle difficulty -- you know, since it happens during battle).