What units to use as games goes on

[DGDobrev]

1) Damage dealer is any unit with decent damage per leadership ratio and is usually a unit that will not take damage in the fight unless it has to. Such units are (in my opinion):
the Horsemen when having enough space to charge
the tough units, who can also act as tanks if need be (Paladins, Knights, Black Knights, lvl 5 units)
the units with "No Retaliation" ability (Royal Snakes, Imps, Fairies, Dryads)
the ranged units when there's a good tank with the spell Target on (elves, hunters, skel archers, etc.)

Those are the ones I usually choose to mop up the enemy.

2) That was a figure of speech... The Royal Thorns and other summoners simply spawn units that can be lost without worrying about them. So if your tank remains behind and your expendable units charge, they will draw the attacks of the enemy troops, leaving your tanking stack safe for a while while your ranged units take care of the enemy or you cast spells to buff up your tank. Having your Knights or Paladins start tanking with stone skin on while all other enemies are burning or otherwise with reduced capabilities or numbers is much better than having your tank soak up the damage right from the start.

3) Get distortion magic skill in the mage tree. It will allow you to scribe the spell into your book and use it whenever you wish. The scrolls are limited by their total number in your spellbook, so if you want to use a scroll permanently, do not use the last one! You'll need to copy it into your book later.