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I'm a fan of demons. ;) I can say that the "run around with the hero and attack" tactic isn't stupid at all; sometimes it's the only thing that saves you. Although, you need to calculate the number of movement squares and the turn order down to the smallest detail. In general, it's incredibly time-consuming, but effective.
If you're playing as Deleb (or any other engineer), you need to get Fire Arrows as soon as possible. Then clearing out neutral creatures will become easy and enjoyable.
I remember killing 20 unupgraded Nightmares with 20 unupgraded Demons. I positioned them like this: 5 demons in each corner, and five diagonally. This is how I always protect ranged units from four-square creatures.
And the ballista quickly and quite painfully eliminates everyone. It's also lucky when you find a Marksman or Spikes in the guild.
After upgrading Succubi, everything goes almost like clockwork, because Imps are primarily good not for taking control of a unit, but for blocking its attacks on your warriors.
As for Elves, I can say that you shouldn't bombard the enemy with a swarm of Dryads. You can, of course, divide the Dryads into many stacks for greater efficiency, but if there are already about a hundred of them, it's better to just fly in and attack. And if you take Nymphs (it's strange, a Dryad is a subspecies of Nymphs), they already have quite decent damage, and they quickly destroy everyone. Pixies are a cheat. ;)
And yes, you need to upgrade Marksmen as soon as possible and you can't lose them under any circumstances. Therefore, in battles with Stalkers or Assassins, it's better not to deploy them at all. By the way, recently I've been disappointed in the blue Marksmen. I'm more often upgrading Elves into the good old Archers.
And, Waishan, it's better to have fewer troops, but be more upgraded. Higher level magic, stats, and skills. Remember, there's even a sad story in the tavern about a Warlock who won battles with great losses, but was well-upgraded.