Homm5 Skill School

Splitting into single units is justified if:
a) you simply have empty stacks (at the beginning of the game, they are naturally empty)
b) if you want to level up a second hero and have a non-zero army for him right away.
This is my approach, but I would be glad if experienced online players share their opinions.

Single units are used to cover archers/the main stack, to absorb return fire, and to distract neutrals.

- regarding tactics: is there any point in sending a strong unit directly into the enemy ranks to deal damage (but it will be killed immediately) - for example, djinns?

Almost never. Djinns are casters, not damage dealers, especially when fighting neutrals.

how can mages attack dragons without taking losses =[/QUOTE]
They have powerful mini-units. The same ballista will be effective, obsidian golems, healing with a wall of fire, etc.
I've heard a lot about hit-and-run tactics, and I've seen it myself. But I can't understand how to do it (for example, with Warlocks) - do I need to level up Mentorship and teach other heroes Armageddon?

Firstly, it's not necessarily only with Armageddon. You can also use a regular Lightning Bolt. The name of the technique suggests that you also need to run away after casting, not die. Therefore, the main hero can also be used.

is it worth attacking strong neutrals with losses (but gaining experience from them) - or is it better to only attack those you can 100% easily kill and move towards the opponent?

What matters is what they are guarding. To give up some unnecessary units, but get a valuable artifact - attack.