There isn't one answer to successfully defeating Mission boss fights. Each boss is different and each player's vampire build is different.
Mission Bosses have their own attack and defense and are able to land critical hits.
Each time you hit, the boss defends and hits back.
Depending on your build, you might find that you're not ready to defeat a boss when it first becomes available.
Many times the best answer is to pass up the boss that's too difficult and come back to them later when you're stronger.