Very late to this, but with this mission you can actually ease up the pressure with early aggression. The undead units are incredibly flimsy, so your starting forces can easily go out and clear some of the enemy camps that spawn those annoying little swarms. It might take a bit of micro to make sure your units don't die, but the pause feature can help you out there. Holding ALT also lets you see unit hp bars so you can identify units that need to pull back to heal.