Considerations.
Our attackers seem to closely match the
description that we have of the pirates on the Sava; "three boats full of ragged-looking men armed with spears and clubs". Given our current location in their area of operation, and the fact that they're hitting us in a location where they could have been feasibly lying in wait for anyone walking along the shore or floating down the stream, I'm inclined to say that this is a pirate operation rather than a directed attack on us. Ezti is the only one here who would likely have a targeted attack focused on her, anyway, and it would take a very good intelligence-gathering effort to determine that she was going to be walking along a border that isn't even hers.
This lets us know something about their motivations- they're in it for money, and likely to be concerned about remaining safe and maximizing profits, rather than anything else. It lets us know that they're more than willing to kill hapless innocents in pursuit of that goal, so their threatening or hurting Chesna in order to force our compliance is a very real risk if they get half a chance. It also lets us guess at something about their numbers; at minimum, their group can fill three boats. How many men are in a boat? I have no idea. I'd guess somewhere between four and twelve, so we could be seeing all of their group or less than a third of it. Dominic might have a better idea in-character based on the size of the markings on the shore where the boats were clearly pulled out of the water.
As for effective responses...
I have serious concerns that using fire sorcery will take concentration that we can't afford. If we were more skilled in that field things would be different, but we're no human flamethrower- a torch flame is the best we've got. While that's more than enough to bluff with, it's nowhere near enough to add an effective fire component to a shock and awe type assault. Worse, it'll take concentration to do two things at once; we haven't done any kind of training with working fire sorcery into our combat style, so far as I know. The same applies to force sorcery, not that people are recommending using it; in the future we might want to train it up a bit and learn some kind of deflect arrows application so that we can ward ourselves and others from ranged attacks in situations like this, but we certainly couldn't manage that right now.
Flesh sorcery, however, is a different story. Not only are we more skilled with it in general, we've actively practiced using flesh sorcery to boost our strength and speed for situations exactly like this one. It's likely that it will require less focus on our part to use, and frankly boosting our speed so that we can close to melee with the archers supernaturally fast is likely to be at least as intimidating as the tiny bit of fire that we can manage. If we use magic, it should be flesh sorcery boosting our prowess.
Speed of response. The pirates/bandits have already snuck into ambush positions and seem to be holding in place. Our group, on the other hand, is surprised. This means that it's to our benefit to buy at least a little bit of time, so that our forces can get their bearings. Going charging into the enemy right away would leave our men at arms rushing to catch up; hardly ideal. Our discipline and skill will tell if we give them a chance to, so let's buy a few seconds.
Consider talking. We aren't going to surrender, of course, but we can at the very least attempt some intimidation while our forces ready themselves to charge and get Chesna into a spot where she's shielded from arrows. Project confidence, speak forcefully, and tell the bandits they've made a mistake; we aren't the usual peasants they make a practice of robbing, and because of that there's simply no way they're getting out of this one intact. Either they rob us and we send troops after them later, they kill us and our families send even more troops to hunt them down to the last and see them dead, or they fail to rob us because we fought back and showed them exactly what the difference between a true knight and a man with a spear is. If they've got a decent mind between the lot of them they'll realize their mistake, stop this ill-considered assault on their betters, gather whatever belongings they have between them, and be long gone before we return with a few dozen troops to sweep the woods and clear them of criminals.
Talking serves multiple purposes, of course. It'll hopefully intimidate them into withdrawal, failing that might impact their morale a bit, failing even that it will provide a distraction while our troops get their shit together and we slowly close toward melee range, and make them focus on us instead of the ladies. If we talk loudly and forcefully enough to dominate their attention it might also stop them from effectively giving orders amongst themselves.
It'll likely come to fighting in the end. When it does, it will be vital that we prevent any damage being dealt to Chesna- everyone else here can likely take care of themselves, or mostly so. Drawing attention to ourselves with a charge toward their bowmen will hopefully prevent them from shooting at anyone else but simultaneously make us a hard target, and carving through their archers before the melee troops can respond would be an ideal opener. They outnumber us significantly, but we can probably take on several scruffy bandits, and their morale is likely to break if they lose even a handful of people- based on their surmised identity above they're successful enough pirates that they aren't utterly desperate, and they haven't been so thoroughly bribed that they'd risk many casualties for victory. Try to encourage them to flee via intimidation at any chance we get, and let them go if they do- the important thing is to protect our own here, not go clearing out lowlifes. We'll return to do that later with more troops.
Note that if for some reason they are willing to risk significant casualties and have many more troops here than we can see, we might be simply unable to win- being overwhelmed by numbers is a very real thing. If things seem to be going in that direction, protecting Chesna and Ezti takes priority and we need to be willing to shift our objective to "fight our way out" instead of "force the enemy to flee".
Hopefully we'll get through this okay. Would that we had more healing abilities; I strongly suspect someone on our side will be hurt before it ends.