Invasion of meridell can be quite a difficult game if you do not think things over and properly prepare yourself. With these few tips you will be on your way to beating all of the missions with relative ease.
1. Always go for the items on the earlier missions and mobilize all of your troops so you can get them with some decent attack/defence strength.
2. Always battle near the potions at the center of the level so if you are low on health, one move will take you to a potion.
3. Once your skieth has the amulet, your in the good and you will have an easier time beating the enemy. On the first turn, teleport your skieth so it is able to attack one creature and only one creature may attack back, this will interrupt the movement of the enemy and allow you to get your troops in position. Once you save a skieth invader on the third mission then you should get him an amulet and get rid of your moehog. I strongly recommend two skieths with teleport amulets, they will win you the battles.