How do you like the idea of leveling up a hero? It’s quite possible to reach at least level 15 – there are weak units in the underground tunnels...

But in my opinion, it’s not necessary; you can manage with Fedor. However, you might have an unlucky leveling build, so you’ll have to come up with something clever or replay the campaign. Personally, with the following build: Expert Leadership (I don’t remember the secondary skills, but they aren’t important), Expert Luck - Soldier’s Luck - Elven Luck - Magic Resistance, Expert Light Magic - Mass Blessing (or whatever it’s called, the one with speed and punishing strike), Expert Logistics - Native Lands, I didn’t have any particular problems.

The advantage is that both undead and your own troops will be involved.

Regarding undead troops in an elven army – it’s very effective to replace Dryads or Dancers as lower-tier troops with a bunch of Spectral Dragons as higher-tier troops.