Did you play the original Demeo game much? (I'm mostly asking because I'm curious how familiar you are with all of the cards and character abilities)

I would say that level is a little more frustrating than a lot of the others, based on how they keep refilling their armor. (I was glad once I finally got 3 stars on that one) I think most levels after that one are more fun to play. Here's a handful of random suggestions for that level...

1. I usually start with both of my playable characters next to each other off to one side, and then I try to have both of them attack the same enemy (killing one will make the second Guardian easier to manage, try not to let them both heal over and over again).

2. If you aren't familiar with the Demeo cards yet, the Hunter's Mark card should help you get rid of one of the Guardians. So don't forget to use that, which will then double all damage done to that enemy (until they die, it will reset once they respawn)

3. Keep in mind any time you setup a turret, or summon anything, that will improve the visible area for you, which should make it a little easier to keep an eye on where the Guardians are located. So don't forget to setup the turret, summon any monster cards you might have, and also don't forget to setup the decoy, especially early on, because if you can get the Guardians to waste their spears on the decoy, that is a LOT less damage they can do to you in the opening few rounds. (Just make sure you don't stay lined up directly behind the decoy, try to set it up off in the distance, and then you can snipe anyone who surrounds it with your Hunter)

4. It's annoying that the Guardians can heal their armor every round, but don't forget your Guardian can do the same, so if he takes a few shots, you should be able to keep his health at a reasonable level, by refilling his armor every round (assuming he has taken damage, otherwise use both of his turns to attack people)

5. You mentioned the two main enemies do TONS of damage. Don't let them scare you TOO much from their first shot. I think they each have a spear that they can throw once, so that's why I suggested trying to get them to fire that at the decoy if possible. Once they throw that spear though, I don't think they get a new one for the rest of the level, so as long as they don't get you with that, they shouldn't really do any more damage than any other main character in the game.

I don't have much advice for your two follower companions on that level, I usually just leave them alone, and let them do their own thing. I also try not to spend too much time killing monsters, and I try to focus most of my attention on the two Guardians, unless you can't see them, then you might as well shoot whoever you can see.

There are also some strategies where you can try to lure them into the center, and then set off the purple void lamp in the center, to catch a lot of the enemies in a poison cloud, which can help.

That's one of the things I love about the Demeo games. They can be frustrating, but there are tons of ways you can approach any given scenario. Hopefully a few of these suggestions might help you out with that level.

As for the stars, I don't really know a definitive answer to that question. I have all of the stars on the first 17-18 levels or so (there is one I only have 2 stars still) I mostly just try to get 3 stars to extend the gameplay, and I'm assuming you get more keys when you get all of the stars, so I wanted keys to unlock the other characters and a few skins. I don't think playing the same level over and over does much, other than give you some practice with new strategies. (And then of course if you get more stars than you did the first time, then you'll get some more keys) but replaying levels you have 3 stars on doesn't give you anymore keys or anything as far as I can tell. Maybe more experience? So I guess that's probably worth something. The whole leveling up system in Demeo Battles feels more convoluted than the first game. It mostly just makes more stuff available to unlock eventually once you have keys to spend.

Good luck!