That depends, if you give fortified mind
-to all brothers
-to a few brothers
-to no brothers
I like to give all my brothers fortified mind, because Lost Souls are otherwise so very annoying and require special tactics to deal with them.
First, lost souls will always try to scare your brothers before attacking. Even if they are in melee, they’ll try to howl. But only if brothers without fortified mind are nearby and not fleeing. That’s why I sent out my brothers with fortified mind first and let the others follow closely.
This way the first line deals with the lost souls while they try to scare my second line instead of attacking.
This will work best in a line-battle. But what if you are surrounded?
If you’re in a forest make use of the trees. I could be wrong but I think if the lost souls can’t see a brother, they cannot scare him.
General stuff:
Maximize your melee defense. Armor doesn’t help at all, if you’re only fighting lost souls. No two-handed weapons, except billhooks and such for attacking over two tiles. Bows are pretty useless, unless you have a true master sniper.
Shields, shields, shields. Spears are good, too, since they hit easier (I think?), and all you need is a single hit anyway.
Also, it’s not a Bad idea to invest in hitpoints, too. Sooner or later hitpoints will be more important (with the next update). Of course that won’t be save game compatible.