Yeah, I attempted it again earlier and this time I stuck close to Alphinaud and discovered that if there are no more enemies your group of allies are fighting, they are scripted to move on to a specific area and engage the next lot of Garleans, so I made sure to keep close to them when they moved, fought what they were fighting, and eventually they arrived at Fordola's group, and with several healers backing me up, Fordola and her thugs didn't last long.

Ironically Zenos gave me no real trouble at all, other than I barely took any health from him at all and had to heal myself with Clemency a few times throughout the fight. I think I knocked down about barely 5% of his health before he finally said "I misjudged you. Time to end this!" or something and ended the battle. So don't get discouraged! Keep close to the party in the first part, and just keep up the heat on Zenos in the second and you'll finally pass this (given what happens, I stop short at calling it 'winning' however.).