Well since most of the points are pointed out all I can add is…
Chose your enemy wisely, it is better to let those orcs to raid a village than to be massacred yourself. It is no shame to run from the werewolves when they clearly overpower you. Don’t be the hero you are not, if you want to get rid of bandits in a region look around in villages for proper jobs so you get even more out of the fights. It is not shameful to accompant the guards and fight on their side, you will get the experience and even loot a more safe way, besides you get the feel for bigger battles that personally I find quite nice. You are a mercanary in a deadly world, why take a caravan escort mission throught an enemy territory when you can take the other one that is safe and has a low chance to be ambushed.
In battles shields save lives, get them, get them all. Don’t shot the enemy who is surrounded by your troops, most probably you will hit your comrade in the back, perhaps even kill. When equiping your sellswords, and leveling stats, do try to have a minimum of 60-70 fatigue so you will be able to execute at least 2-3 special weapon skills like spearwall or riposte. Stamina management is the most important, going heavy armor as soon as possible is not the right way, you will end up without stamina just a sandbag for enemies to punch. You can test your stamina management againt undead armies, huge wiederganger hordes will show you all the errors you still have, be warned tho they are deadly if you run out of stamina.
Have back up equipment, since the weapons and shields break in battle. Nothing is worse than to be in the perfect spot with a broken weapon. Even at the worst cases, remember you can pick items up from the ground! You can freely switch between items in inventory at any pont, doing so will cost you 4ap / item if you do not have the utility perks. Especially important in long battles against armored enemies as orcs. Always remember, here are no predestined character classes or roles, you can always have a weapon set for a different role or purpose. Do not have axes to destroy shields, use flails to igneore them instead. Archers are making it hard for you, switch to back up tower shields. Spearwall done it’s work but now you neeed damage, get out that other weapon of your belt. Archers ran out of arrows, it is ok just swith to the other quiver from your backpack. It is one of the things I love about this game, you can play as you want without limitations, most items and skills are viable or you can makem them to be.