This is where I'm stuck at now, too. I've cleared the entire map, found all of the hidden items, but I can't figure out who to use to kill each of the bosses. My party consists of:
Eirika, Joshua, Gerik, Moulder, Vanessa, Seth, Colm, Ross, Lute, Saleh, Amelia, and Neimi / and, of course: Ephraim, Knoll, and Deussel.
I can hit Valter with Saleh...to which his HP goes down to half, but then he criticals Saleh -- BAM.. dead. I just can't have it this way. I'm a pass each mission with all party members alive kind of player -- or else it's FISSION MAILED.
Help?
That's not a terrible line up. You have one Flyer and two Magic Users who are reliable, so that is a good thing. The biggest thing on this mission is to focus your attention on 3 things early on:
1) Defend the Right and Top sides of the map. Wryven Riders come at you from the right side and Pagasas Riders come at you from the top. It's nothing that Neimi and Lute can't handle, so just have those two playing defense with maybe another unit like Colm, Amelia, Deussel, and Knoll (Once they make their way over to your stronghold) since they can't move very far in this mission and are better to play defense with anyways.
2) You want to keep Ephraim and his little squad alive and there are two ways you can do that: First is to just hold your ground until the chapter is over or the second is the focus all of your efforts to having him join up with your main squard (Which I will get to in the next step). I would recomend the second opition over the first since A LOT really hard to kill guys will spawn my Ephraim eventually, so it's better to just focus his little group to join up with your main one.
3) Establish a main traveling unit and have them move as a giant group from boss to boss. In your case, have Joshua, Gerik, Seth, and Ross take the front lines of the group to meet the enemy and take the hits. All of those guys either have pretty solid Defenses or they have no problems avoiding hits. Behind them have Erica, Saleh, and Ephraim as clean up crew to either finish off easy kills or kill off anyone in the way of the front line moving. Behind the Middle Men have Venessa and Moulder to help with Healing. In tight spots, Venessa can rescue your injured member and litterly take them to the back lines where Moulder can come in safely and heal them.
Once you have established this set up early on, it's pretty much a cake walk from there. Just remember to take it slow and stick to the layout I have given you and you should be fine. Just be sure to keep track of your health and don't be afraid to us an entire turn to have your guys use healing items. Remember you alway want to play Defense first in this game. IDK if you are having problems with the boss on the left upper corner (Name escapes me at the moment), but just have your little group travel that way first to meet up with Ephriam's gain and beat the boss, then work your way to the diagonally towards Vaulter and by the time you get there, you should have only a few enemys left, that or only have him left to beat.
The key to beating Vaulter is to use magic units on him since his stupid item makes bow users useless (I hate that about him). If you say Saleh does the most damage, use him, but it seems like you don't know to exploit bosses yet, so I will give you a little tip on how to beat him easliy:
First, have Saleh attack Vaulter from a distance. Saleh should have the advatage from a far. If Saleh doesn't take damage from Vaulter, use the end turn command and allow him to attack Saleh (IDK if Saleh your build of Saleh can take a single hit from Vaulter but if he can't, don't do this obviously xD). After you have taken damage, use either Seth or Vanessa to rescue Saleh. After they have rescued him, they should be allowed to move again (A perk of the mounted units). Move them back away from the attack range of the boss (They don't move, so don't worry about being chased). Use whatever unit you didn't use to rescue Saleh (Venessa or Seth) and have them go next to the unit who rescued him and select the "take" command. Right after, select drop, pick a tile, and there you go, your unit is out of attack range. You can then use Moulder from a safe distance without having to worry about taking damage.
I hope this helps and just remember to think defense first and take it slow. Good Luck