Spread your damage out among your enemies as much as possible. Reducing the health of an enemy unit will also reduce the amount of damage it can do. When ready, give priority to taking out the weakest units first to reduce your enemy's overall damage output and quickens the appearance of Pillage mode.
Also target an attacking unit's armor. Although eliminating a unit's health will kill it, reducing their armor first can make this task easier. The damage given out equals your strength minus your opponent's armor. Armor also increases the chance that your opponent will deflecting all damage (the chance to do so is 10% for each point of armor higher than the your strength).
Use heavily armored melee units (for example, shield-bearers and Varl) to block enemy attacks from your vulnerable ranged units. With shielded units, line them up together and use their blocking ability. Also use this blocking strategy to protect resting characters.
Keep your ranged units towards the rear, and away from melee attacks. Maximize your archers' Break stat to allow them to soften up the enemy easier.
Use units that have Heavy Impact (for example, Bersi and Hakon) close to enemies that are clustered together.
Ludin's Impale skill does extra damage for each step the enemy takes. Use it against Dredge Slingers because they always take a free movement action after being hit.
Use Willpower as needed to quickly move a unit out of trouble, activate a special skill, and add damage to attacks. It is restored to full before the next battle.
Use the right-most icon of the Horn symbol at the top of the screen to give one unit of willpower to your current unit. Because the Horn only holds a set amount of Willpower, use it regularly so it will not be wasted.
Characters with useful special abilities (for example, Yrsa's area-of-effect attack) require more willpower.
A maxed out Exertion stat allows units such as Varl to put more Willpower into each action.