i'm on chapter 3 where vann, by himself, has to sneak past 3 guards to a secret entrance. and vaann must survive. i keep getting killed at the first guard. can u help me and explain how i can get past the guards when i have no team and no espers?[/b][/b]

Wacth the guards to see their patrol routes, then time your runs so that you run past when they're not looking. if you cast sprint, it'll help a lot.
Stay well clear of any berserked enemies.
Slowly make your way to the far corner (go around the central path, not along it, there are too many enemies on it). Then when you get near the entrance, just make a break for it, and run straight for the doorway, trying to avoid the guards.
If you fail, just try again. It took me 5 or 6 tries.

I had a completely different but effective approach. After several attempts (all of them fails) i just used Sprint and ran straight to where you're meant to go, and it worked!