"Never underestimate your opponent" – this rule is at the core of the witchers' code, and for good reason. Proper preparation has saved many a witcher's life – it is likewise worth your while to brew potions, concoct oils or visit craftsmen before engaging in combat.
Blacksmiths forge weapons, while armorers craft armor.
If you expect to battle a strong opponent, you might want to complete some side quests, contracts or treasure hunts before attacking. This will allow you to gain experience and develop your character, giving you a better chance of winning the fight.