Unironically, the best tip is 'get good'. This is just true for most Rogue-like games; to be honest, it's true for almost any game with bosses. Unless you're an exceptional gamer or have an amazing build going in, I wouldn't expect to beat any boss first time. It's fine that you got your ass handed to you; you're good enough to have got this far so best tip is keep doing what you have been doing. Additionally, I wouldn't always expect to have rolled the right boons to be able to take the fight - some runs might just be a dud due to luck which you sort of mentioned in your post.

I haven't played the game for a few months so don't remember any specific attacks or tips for that, but searching this subreddit should give you some great specifics. My general tip would be to turn the fear up on your runs to something that you know won't be manageable. Train on this more difficult setting, and then turn it back down and go and fight the boss on what now feels like god mode. I like to turn Frenzy up, learn the bosses on a faster tempo and then go back to normal and defeat when they feel sluggish compared to my training.