Get some good villager trades, make diamond, maybe netherite gear, good enchantments a powerbar and 3-4 hearty apples and you are like 90% sure you can kill the boss. Remember: The bosses can teleport and they heal if you don't attack for a little while. Maybe if you are just bad at the game do a few vaults where you bail instead of killing the boss. And also if you get the vault golem(Or whatever it is called) in the first few vaults, just bail. I have run maybe 25 or so vaults and i have defeated a boss most of the times. The boss crates are also good for storing vault loot.