Sworn enemy.

And if in this case you try to play defensively, take, for example, Preparedness. It will be dangerous, though, if the opponent has dark magic (although the Elf has plenty of resistance). Or there is an alternative: put Implosion, Frost, Fireball, Stone Skin, Ring of Cold, etc. on the arrow. Although Meteor Shower is the most suitable and most powerful spell.

IMHO
Playing defensively is not beneficial for the Elf. Skipping the first turn (because you are in defense) leads to too much damage loss (which fairies + dancers + uniques + dragons can do in their first turn, with the right hero build and combat tactics, I don't think I need to explain). Good resistance only works when the troops are standing next to the unicorns, and this is an excellent target for the opponent's mass spells (even considering that half of them will not go through).
Against Heaven with its well-fed units and immense defense, this tactic may be useful, but against other races, it is not worth experimenting with (or if you are playing with a friend and you care more about fun than winning).