This option intrigued me, so I decided to test it. I took a level 5 witcher, gave him basic artifacts (which is quite appropriate for his level), and the First Strike spell for backup. Of course, I didn't give him Adamantine armor and a Rune Axe. When attacking 100 griffins, the witcher would fall on the first or second hit, without taking down a single griffin, even though the skill was active. This seems inconsistent.

Did I promise that the battle would be easy? The First Strike spell is the main one, not just for backup.
The battle tactic is important:
1. On the first turn, cast First Strike - обязательно!!!
2. Don't move yourself to the edge of the map - stay in the center so that the griffins always have somewhere to run.
3. Don't forget about positive morale; if they attack with morale, we won't respond - use the Scroll of Sadness - cast it on the second turn, and they won't attack us then, or wait each turn and attack during the negative morale phase. As an option, use Confusion, but I haven't played Heroes for a long time, so I won't indicate the correct moment to use it now.
If you do everything correctly, the witcher can take down an infinite number of griffins, unicorns, devils, etc. (in such a battle, the computer won't summon ice creatures with the devils)