I'm still not sure if they'll have a Sending and/or Teleport available, so anything's possible on how this could go. The party oracle doesn't have Sending as a known spell, but he could pick it up on this level up, just like the sorceress could grab Teleport.
We've actually got a relatively "normal" party composition - one arcane, one divine, an archer, and two front liners. It's just that the arcane and divine casters are both spontaneous (sorcerer and oracle), so they don't have as much flexibility in what spells to prepare as wizards and clerics.
I'm thinking if they do go the route of Sending to Sandpoint to warn them a week or more in advance, I'll have only a small percentage actually leave. Nobody's seen a giant or dragon anywhere near Sandpoint since long before Magnimar and Sandpoint were founded, so they're likely to be skeptical of the warning, even coming from the town's local heroes. Maybe they'll send to Magnimar for help (which will get caught up in corrupt politics and not arrive in time), and send out some scouts to provide advance warning if giants are seen in the area, but at least half the townsfolk will still be there for the attack.