Well, let's try to summarize. Fear: living creatures don't respond to the attack and retreat slightly. This results in an unreturned strike during the attack. However, retreating doesn't skip a turn, and the enemy also attacks, and after receiving a counterattack, retreats again. The attack and counterattack happen simultaneously, and the Scarecrow may take damage from the enemy's strike (I haven't tested this).
The First Strike ability allows you to strike the enemy first, regardless of whether they are attacking or defending. Therefore, it's natural to cast First Strike on the first turn. This should also prevent taking damage during the enemy's attack. The enemy doesn't necessarily need to retreat from fear – after all, it's not a ranged attack that can be blocked. In this situation, morale only affects the turn order (which doesn't matter for the first strike in a 1-on-1 situation) and the attack strength (which also doesn't matter for fear). Morale doesn't skip a turn. Therefore, neither zeal nor sadness is useful (zeal is still helpful – we'll deal more damage and finish the battle faster).
However, confusion or any similar spell would be very helpful (even without First Strike), but where can you find the guy who will feed two generals? Because if the Scarecrow casts a spell, it can't attack.
If the enemy has 2 or more attack slots, fear won't work on the 2nd and subsequent attack slots. And then the Scarecrow has no chance of surviving.
Summary. The Scarecrow with First Strike can win against a regular melee unit, even on auto-battle. Nevertheless, it doesn't win. What's the problem?

"The computer doesn't summon ice creatures in this battle with devils."
The computer is clearly being slow. It often summons ice creatures, but here it seems to think it can defeat the enemy with a single blow and doesn't bother.