I found a way to win the quest “A Bargain is Struck” very quickly and easily, even on difficulty level “Brutal”.
Spoiler alert! Don't read further if you don't want to read my strategy.
At the beginning, immediately walk with at least four miners or other non-critical units down the west side of the river. At the same time, move the caravans and the gnome duelist north to the river.
You will find a raft there.
The idea is to use this raft to cross the river to avoid any combat against the kobolds.
You have to cross the river several times, but the single raft will do:
Put a miner on the raft and go on the other side, then make the miner walk to the land, then put one of the mission units (gnome/caravan) on the raft, cross the river again, then let it go to your mead hall. Now you just need to repeat this 4 times and you won without any battles!
On Brutal difficulty you may want to make sure you stay far away from enemies, but it should be doable if you are quick and careful.
My comments and suggestions on this mission:
I think this strategy basically breaks the mission, since it makes it quite easy to win. Was the placement of the raft even intended?
I suggest to remove the raft on difficulty levels Normal, Hard and Brutal, but keep it on Easy for fun. :-)
Also, the mission objective was not clear. The objectives fail to mention that a) the kobold duelst must not die and b) the kobold duelst must reach the mead hall.
Also, when the kobold duelist reaches the mead hall, he claims he will “stay for refuge” (or something). I would assume this means the unit disappears from the game, yet it is still in it.