You cannot cast level 1 last hero on a Horseman, you need it level 3. Also you can't cast on a level 5 unit like a red dragon for later in the game. The stronger and fast level 3 unit is probably a griffin, that requires level 2 last hero.
I'm not sure one griffin alone can attract the attention but that's definitely something to try later in the game. I was also thinking of trying that later in the game with a reserve stack to use a one unit stack or a full stack depending of the battle.
For a mage with two casts it's to combine with a target spell. That could change the face of many battle in a way a brute warrior couldn't dream of.
