Heres what worked for me:
Make sure Adren has the emerald of healing from the previous mission as this allows you to cast healing wave and keep your units fighting longer, also if possible get the granite figurine item (by beating Hydrus the destroyer 3 missions earlier) as it let's you summon a granite golem. When the mission starts get all your forces and rush the northwestern base, while you do this make sure to research all the upgrades, although their hero has reincarnation this shouldnt a problem because he starts at level 5, it might take you a few tries but your starting forces of swordsman, archers, and knights should be able to wipe out that base as long as you command them right and focus fire, it might take you a few tries but with any luck you should level the base and still have your hero and 2 or 3 units remain. At this point you're gonna have a much easier battle; now you have less enemies, thus less attack waves to worry about, you got yourself a 2nd goldmine, also the base has a merchant shop in the back so you can buy some good items. With that being done the mission gets much easier, sure you still got 3 bases to go, but trust me it will take you much less than 2 hours.