I take High Elven Dragons because of Sorrow. I understand that High Elven ones aren't fatter, but even if I have 100 of them, and the difference in HP is 10, 1000 HP won't save me. It's already clear that I have the Cloak of Shadows. And I won't find a second artifact from the set. He already has the Staff of the Underworld and the Ring of Holy Words. I hope I can get the Ring of Swiftness.
p.s. He was unlucky with the Rune of Energy and the Rune of Etherealness. Which I'm very happy about^^)
The only thing that bothers me is my constantly unlucky attacks, which await me, and the lack of Soldier's Luck. But I wouldn't be so categorical about summoning with Helmar. An Imp on non-High Elven dragons is a nasty thing. Considering that his dragons' stats are twice as high as mine, even without magical enhancements.

Added 4 minutes ago
Essentially, the point is to overwhelm his creatures' stats with debuffs to the level of my own and overwhelm him with numbers.

Added 10 minutes ago
I also wanted to ask a question about the arrangement. (I mean the tactics, as described above)
Here's my version:
Bottom left corner - Archliches
To the left of them - Zombies
To the right of them - Warriors
To the right of the Warriors, in the top row - Dragons
To the left of the Dragons, below - Vampires
To the right of the Dragons, below - Spectres
And to the right of all this, in the bottom row - Messengers.

Please correct it. I have vague doubts about the Vampires... should I hide them and keep them until the enemy's creatures are reduced to a minimum, and then throw in the Imp and Vampirism on them, or should I put them in the first row and hope for Stun and also that they won't die from his first attack?.. I'm waiting for comments)

p.s. Thank you very much for this article - http://heroes.ag.ru/heroes5/advices/11/ I learned something new))

Added 2 hours 53 minutes ago

The only thing is that Helmar is a complete weakling against...

Against what?