I am sure this is because I probably wasted too many turns learning how to play instead of making optimal choices and how all important enemy stacks come with stronger value than what I would level up over long time.
I'm at around week 30 on Balanced Alchemist, and I to had to go through similar issues, only I have demons instead of the undead. They had stacks of t3 units with some t4s when I was happy to find any t2s other than the gobbos. My advice is to relocate to greener pastures and get some questing and leveling done there, away from your main enemies. I did that, and now I'm back with a vengeance (also running away from some crazy dwarf bitch and the guy from the first Spellforce who for some reason is the orc lord here).
This is also the biggest problem with the game, all enemy stacks in the game become stronger over time while you don't really have a good way to level up your new units. Also always try to keep one Hero close to your tower if you need to come back to defend.
You can shuffle your units around, give some strong ones to the fresh hero and let your strongest one train all the noobs. Also, I've been doing trials by fire with getting a bunch of new units, going into a few battles and if any of those survived, building a squad around them. And if you get the ability to summon a golem - that can help too. 4 or 5 mana per turn is nothing, but it gets you a t3 unit with over 200 hp and infinite retaliations. Though I'm not sure if it gives you a random golem, because I summoned a fire one, but I saw stone ones in the world that are immune to physical damage. You get one of those and you can tank just about anything.
And as for defending - town portal goes brr. I think it's one of the basic spells that everyone gets, so you should consider researching it. No need to waste a squad on defending.