Sworn enemy.

In my opinion,
it's not beneficial for an Elf to play defensively. Skipping the first turn (because they're playing defensively) leads to too much damage loss (which fairies + dancers + uniques + dragons can inflict in their first turn, with proper hero development and battle tactics; I don't think I need to explain this). Good resistance only works when the troops are standing next to unicorns, but that makes them an excellent target for the enemy's mass spells (even considering that half of them won't go through).
Against the Haven faction with its well-fed units and immense defense, this tactic might be useful, but it's not worth experimenting with against other factions (unless you're playing with a friend and you care more about having fun than winning).

...To be honest, I don't really like developing an Elf as a Chaos Knight. And I don't prefer defensive tactics, but it can still be useful.

Against the Order faction, I can't even imagine how you could play defensively; I think it's only possible to play offensively. Relying on archers is dangerous; either Blindness, Slow, or Haste will be cast, along with other nasty spells. And those damn new crossbowmen sometimes annoy me with their damage, although their initiative is low. And you can't let the paladins wander around the battlefield too much (especially the alternate paladins).
I think a defensive tactic + Chaos against the strong, heavily armored Necropolis units could work. Against them, it's generally very profitable to stand in a defensive position and bombard them with Chaos spells (Meteor Shower or Ice Blast) until you run out of mana. They are slow, and no one reaches you immediately. Mass debuffs and high-level spells, like Implosion, are effective (the only problem is Haste, which can't be removed with Cleanse; only with Antimagic, although there's a chance of resistance). Therefore, there's a chance to hit them with Chaos 2-3 times before they reach your troops. After that, it's all about the attack of the Elf, the Necropolis troops are battered, and all that's left is to finish them off (if you have mana left, you can take Magic Resistance in attack and use it just before the attack).
I've tested this tactic, and it turned out to be quite effective.

...Damn, I didn't even notice how I strayed from the topic...