I'm playing without losses, 12 Titans, 40 Mages, 400 Gremlins, and some other units. The Demon has 100 red Unicorns, 50 of the second-to-last level, and 400 Hellhounds, with an attack of 20-30 (not damage, but attack). The difficulty level is minimal. These red horses kill any of my units with one hit. After the first round of attacks, I have only 5 Titans left. They are almost unchanged.
I'm wondering if the Demon attacks everyone at the bridge, or if I should replay.

The Demon appears as a task; it seems to be mandatory.
But he is not a problem.
1) As a bonus, use Phoenixes: place them in front of the horses to delay the attack (Phoenixes resurrect, get scared, and return to their initial position).
2) It's useful to have strong summoning magic – the Hive not only stings but also slows down.
4 Hives – the enemy stands still while you deal with them.
3) In general, you need to summon Thunderbirds once a week – the battles compensate for the loss of experience.
4) They join in large numbers – >=60 Mages, Rajahs, Air Elementals.
Good luck