They are tough if your company is young with few/no members who’ve levelled up and gained perks. But, basically, the typical bandit shield reduces the chance to hit by 15%, or by 30% when a shieldwall is formed (it’ll be more if several bandits are in shieldwall together and/or if they are on high ground). Additionally, all Bandit Raiders (unlike Thugs) have a generic 10 melee defence that drops the chance to hit by a further 10%. This doesn’t seem like much, but it can mean that the chance to hit drops from 55% (e.g. striking at a shield-less Bandit Thug with an axe) to 15% (striking at a Bandit Raider in a shieldwall with the same weapon).
A good solution to the above problem is: (a) recruit some guys with decently high max fatigue; (b) give them large hand axes and shields, and a back-up weapon that has an extra +10/20% hit chance (e.g. spears or swords or scramasaxes); (c) use the first turn(s) to smash bandit shields with axes; (d) switch to the more accurate and less fatiguing weapons once the shields are gone; (e) patiently stab and slash away at the bandits’ hit points, and form a shieldwall if their high melee skill gets past your basal shield defence for significant damage.
If some of your guys have high melee skill and max fatigue, you might keep hold of the axes and skip steps (d) and (e). Alternatively, you might give them spiked flails, which ignore the 15% basal shield defence but not the additional 15% defence from a shieldwall (i.e. a Bandit Raider in shieldwall will have only +15% shield defence and +25% total defence against a flail, not +30% and +45%). Flails are quite fatiguing to strike with, and cannot split shields, but their disregard of the basal shield defence means that an accurate mercenary will get some good, damaging hits in (without needing to waste energy on Split Shield) if he keeps striking away (even if each individual hit chance is still only 25-40%).
All of the above strategies are more effective with the following perks:
— Bags & Belts (more reserve items and no fatigue cost)
— Quick Hands (arm with a hand axe only when you need it to smash a shield, and keep initiative higher and fatigue lower the rest of the time)
— Sundering Strikes (light weapons with a higher hit chance can cut through armour more efficiently)
— Dodge (effective if a mercenary has decently high basal initiative and already has some of the Utility perks that reduce fatigue and thus sustain initiative, since this means that you can forego fatiguing defence skills early in battle and keep fatigue low so that you can still form a shieldwall later on if you get hit and need extra protection
— Any of the level 2 Utility perks that further reduce fatigue