Early game use spears as long as melee skill is around or lower than 55. You can eventually experiment with switching weapons according to enemies at hand. For example, you want to throw in some blunts against skeletons.
Most early mercs will not make it to the end of the game so I go for talents that help them survive and deal damage (fast adaptation, colossus, shield expert). If you happen to find a particularly talented merc early on, build him in earnest - always start with Student. All frontliners need Brawny and Battleforged. All archers need Bullseye, Dodge and Nimble (unless you want to build an advanced armored crossbowman/axeman). Everyone needs Recovery, but take that perk toward the end. Quick hands are great for swiss knife backlingers (aforementioned x-bow/polearm fighters). Rotation is great for shield bros, Footwork is a must for 2-h damage dealers and ranged bros. Berserk is good on 2-h DDs and longbowmen. Everything else is not set in stone. I like giving Steel brow to my most precious bros to lower the chances of them dying to a (un)lucky headshot.
As a rule of thumb, aim for at least 120 naked fatique on your main crew. Everything else goes to relevant attributes. You can somewhat neglect melee skill on shield bros. Aim for stronger ranged defence for your squishy back line and melee defence for your front.
Backline absolutely needs to dabble as axemen to break shields when you face noble houses armies, orc warriors and undead legions.
Roster-wise, I'd suggest to start with two ranged bros and eventually get to 4. Initially you'll have lots of shield/spear bros, but toward the end of the game you'll only need 3-4 of them (but super tough). I also employ a banner/sergeant guy to boost morale in certain missions. At least 6 2-h DDs in heavy armor (don't go 2-h on your good mercs until they get to lvl 8 and you can equip them with at least scale). At least 2 throwaway dudes to sacrifice in tight missions (sometimes you just can't avoid losing men, period). The rest I usually experiment with.