No, I don't think it will work. Firstly, the hero will be level 1! What can he do? And secondly, the enemy has thousands of units! Even if I kill 10 of his creatures, it won't significantly weaken his army. :)

If you want to replace your troops with undead, you're planning to gather a comparable number of undead units to match the enemy. And even with a level 1 hero, that number can inflict serious damage on the enemy. Plus, along the way, there might be opportunities to gain experience, which will slightly strengthen the hero. The advantage is that both undead and your own troops will be involved. Of course, this depends on whether you have enough money. The disadvantage is that the enemy might gain an extra level.