The problem is that none of our characters can survive hits from all three enemies. On the plus side, the enemies have fairly low accuracy and AS thanks to their powerful weapons (the mage has Elthunder, for instance). Valor is once again the worst choice because any two hits will take him out thanks to low defenses and HP. Hell, Valor can't even damage the knight, though he'd have the best chance of dodging his attack.
I hate to say it, but maybe Gregor would be the best first attack. He would even gain +2 DEF if he gets dropped next to or diagonally from the Ballista user thanks to not using any MOV, which would boost his survivability dramatically. He also has Daunt, which could help keep him - and everyone else who catches up - alive, if only a little.
Of course, this is all purely hypothetical based on the assumption that A) Haspen allows this, and B) If Seyena can, IC, summon up the courage to charge headlong at a ballista