Well, most of the chapter was me being mauled by arrows and status magic, so I decided after many tries to take it super slow and let them waste their weapons, then I'll slap them with mine. I used Roy, Lilina, Chad and Ellen, Dieck, Lance, Lot, Thany, Rutger and Clarine.
I sent Clarine, Lance, Lot and Thany (I love the delphy shield) alone through the east route, using them as bait for the boltings and berserks/ mute staffs. Once those were spent, I moved them closer to the snipers and tried to take them with Lot and a bow, but I couldn't since my bow broke before their ranged healing saffs did. I moved on through the east and got to the upper bishop with that ranged holy magic. During this whole time I was feeding kills to Thany so that she could beat Sue on levels so I'd go with the Pegasus Knight's route, since Sue was easily my best unit and way ahead of everybody else. I actually managed to put Thany ahead of Sue in levels too, just barely, with Thany at lvl9 promoted and Sue at lvl 8.
After reaching the easter sniper and shoving my axe on his face, the rest of their units started moving, so I had to move everybody that was back in the start of the stage. I tried to feed those units to Roy, since he was still level 19, but sadly Dieck made honor to his name and took most with counters. I moved the upper units to ake the second sniper, and by then the map was pretty much clear. I gave Roy a holy water for resistance, weakened the boss a bit with Thany, then sent him in for the kill. Sadly it wasn't going to be enough (Dealing like 11 or so with a ranged attack from Joyuese, he had a bit over 20; I wasn't going to double attack either) but to my surprise he died anyway, giving Roy his level 20.
Only thing I can think of is that Roy had a full inventory and I was asked to send something to Merlinus when I got the sacred book thing.