If he knows where the enemy is from then why not go on the assault? Create a barrier that locks them in and have it shrink a little every day with an announcement that the death of the leadership will let them be free. If he blocks the sunlight it won't take very long for them all to forfeit when no new supplies can come in, they can't make their own food anymore and with the shrinking space they will kill themselves quite quickly or get squished to death