You begin this scenario at an immediate disadvantage - you're being smoked out of some tunnels. The smoke causes 1HP damage every turn - if you have Regenerate, your generals will be fine. Otherwise, keep your troops close to you, and use Heal when necessary.

As you leave the tunnels, you're confronted by spellcasters and archers, which can very rapidly destroy your troops. Aizel doesn't tale any time in rushing forward and attacking either, so make sure only your toughest groups head to the right. It's difficult, but certainly not impossible. The 500P from the secret tile might also be useful.