We have at least two turns before they get to swarm us. They're in the rear of the train, meaning most of the Wyvern Knights will be hitting first, followed by them. As for any getting past and causing permanent ruin, I would call it very unlikely. Because you'll likely be grabbing the Aeromancer next turn, he'll be dropped behind you. This means no one can get past, since Allies are blocking terrain and you are very, very unlikely to be taken about a Spear. It's a given that if you're going to go down, it will need to be by Pegasus because there isn't anyone else who can do enough damage. On the other side of you, Miguel and Carlinn will be making a wall. Due to the sheer DEF amount of defense alone, they won't be able to break through, regardless of who is in front and who is in back.
Further, if Grifen does take an early hit to a Pegasi, she's still up next turn. We have enough Physic staves to keep refreshing her before her turn. Meaning, no time is truly wasted. She can go back to merrily shooting down the enemy and reenacting Pearl Spear Harbor after she repicks up the Aeromancer. Who will also get a free shot, meaning that whoever in front of you is going to have a bad day, since he will nearly autokill or do a hefty amount of damage to everyone who isn't Vaughn. So, here is what I suggest;
Grif to 7,20, grab her manfu and party down to 7,26
Miguel will ride to 8,26, take Fezz and drop him like he's hot.
The Reverend Betamon will go to 8,28 and restore him. This means Fezz is once more an active threat, which is glorious.
Next turn,
Assuming Vaughn goes to 5,30, the Aeromancer will have likely attacked him from either 6,29 or 7,30. Regardless, Grifen can run to 7,29 and grab him, before cantoing to 7,28. This will set up the kill zone. Anyone who isn't Vaughn will either attack Grifen or the wall on the other side. Anyone who attacks Grifen dies. Period. If Vaughn tries to take her out, he'll fail and open himself up for violent retribution from our defenses. If a Pegasi attacks, they'll be smoked by the inevitably dropped Aeromancer of doom.
Miguel will go to 8,28, dismount, and toss his Iron Pilum to Carlinn who will stand at 8,29. That way, the big man has a way to attack from afar and can cause pain for anyone who hits Miguel. Miguel is a possibility for being downed. But I'm not especially worried about this, since he can take a rather hefty beating. Vaughn is his only real threat, seeing as the Pegasi do chip damage and the Wyverns will only double him after Momentum is gone.
We can see about getting Fezz into the action as well. I think he'll end having to play a vital role when we finally take on Vaughn.