You can kill it if you want; it's worth a whole 20 experience or so. Any troops you lose will get replaced when you finish the training area, so don't let fear of losses stop you.
If you're trying to play through the game without losses, then I'd advise just avoiding it, which is a perfectly good strategy.
In general, you'll probably be a lot happier with your play-through if you aren't afraid to acknowledge when an enemy is just better than you, run away, and come back later with more experience. In particular, there's a few stacks you'll bump into in the first full map which are way too strong for a level-1 hero to even hope to fight. I think that Emerald Dragon (and a few others in the training quests) are the only ones that are "gone-forever" if you choose not to fight them at the time.
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