None of the enemies are OP. They all have several weaknesses and it’s up to the player to exploit them.
Crossbows beat bows, hands down. They hit harder than an aimed shot, cost 5 less fatigue including reload, costs only 5 stamina if you don’t reload, have +20% to hit, and can be fast swapped via Quick Hands for 2 shots or even 3 with the Berserk skill. And using a goblin crossbow gives neat tactical options with its knock-back ability. Crossbows wreck everything except Lost Souls and skeletons.
Goblins – fight them at night, with tower shields, in the forest, or from atop a hill. Use brush to hide your melee and pounce. Take your time and send 2-3 brothers along the outside of the map and attack from the rear at the same time you release your dogs. If you see a goblin with a net, try to kill him before he can throw the net, those nets are incredibly useful against orc warriors.
Orcs – Their ranged capabilities suck so they HAVE to come to you. Pick your ground and fight or just fall back to the edge of the map and retreat to fight them on another map. Use spearwall to give your ranged brothers 2-4 turns to shoot up the soft berserkers or young. Your highest priority targets should be any orc with an axe so they don’t have time to break your shields (use goblin nets to pin axe carrying orc warriors and buy more shooting time). Carry extra shields. When the melee combat is joined, move your ranged brothers into the backs of your men. They’ll have decent enough shots but more importantly the orcs won’t be able to knock your frontliners out of position.
Undead – You only need 1 or 2 brothers with the mind shield vs Lost Souls. Target enemy 2-handers and battle axe wielders. Then make excessive use of shield wall and stamina recovery (front line rotation, the battle horn, doing nothing). The undead are really dumb so have them try to chew on your toughest brothers while the rest of your men swarm the edges. These fights are less about combat skill and more about stamina management.
Bandits – Have fun, they are all different and can surprise you.