Well, I came up with a strategy for chapter 9. However, it would be so horrendously difficult to implement that I've decided to describe it here first and see if anybody else has any better ideas.
First off: Seth has to be brought down to the bottom area. He's the only one who can deal with the boss with anything even resembling speed.
Second: Vanessa has to deal with the northern enemies. She's the only person who can 2HKO the mercenaries (though admittedly she can't double them with anything but a slim lance).
That leaves Tana to deal with the pirates and the lone myrmidon further south. This is a problem, since all three will have to be dealt with as criticals during the enemy phase. This is the main reason testing this strategy would be very difficult.
EDIT: Nitrodon, I just noticed you saying I ought to press B instead of doing check map. It should be hex editable. I'll do that before starting chapter 9.
Note that which order I move the three relevant units in is still in the air and will be decided as I do the chapter.
So, I have pictures!
First off, Tana grabs Seth and moves as far south as she can.
The pirate moves up to attack her, she criticals him. Dodging isn't strictly neccesary, but she can't be afford to be hit more than once overall throughout the chapter.
Vanessa moves all the way forward, and kills the soldier behind her with a javelin, levelling up and gaining strength and speed. The strength allows her to 2HKO to sniper with a javelin, and the speed allows her to double the pirate who'll appear soon with an iron axe.
This is the enemy phase for Vanessa. The picture's a bit confusing, but the idea is that everything that can attack her does. The sniper, archer, and soldier all die, but the mercenary survives.
Tana then moves down and drops Seth off at the filled-in circle, trading to equip his javelin. Tana will probably equip her heavy spear, since she can't kill the archers anyways and countering takes time, but if that makes the archers too accurate it might be faster to leave her with her javelin.
Meanwhile, a pirate reinforcement has appeared at the empty circle. Vanessa moves to the indicated square and equips her iron lance. I don't know exactly where the lower two mercenaries will be at the time, so the exact square may change, but the main point is that she's in the range of every nearby enemy. She kills the mercenary she had attacked once before, as well as the pirate on the counter.
The pirate should have moved straight south the previous two turns, so if Tana moves here and equips her javelin, she can counter him.
And so Tana criticals him on the counter.
Seth moves down next to the boss and OHKOs him with an iron blade critical. Some archer reinforcements have appeared, but I can ignore them this turn.
Two reinforcements have appeard above Vanessa: a mercenary on the right, and a mage on the left. Vanessa can kill the mage in one round without a critical, so she criticals the mercenary with her javelin.
On the fourth and final turn, Tana moves all the way left.
And counter-criticals yet again.
Seth moves to this square and equips a javelin. Some more reinforcements have appeared, but he should be in range of every enemy. At this point he should have 25 or so javelin uses left between the two. Hopefully that will be enough.
More reinforcements have appeared near Vanessa. The one on the right is a solder, and easily killable on the counter, so she kills the mercenary on the left with an iron lance critical.
So, that's the idea. I can't afford to miss even once, and there'll be plenty of times I'll have low accuracy. Tana needs three dodge-criticals during the enemy phase. It's entirely possible that this will require so much RNG abuse that it's just faster to take more turns. At any rate, if anybody has any better ideas before I start this suicidal venture, I'd appreciate it.
EDIT: Apparently pirates use lances in typo-land. Also, left and right are backwards.