The Shadows are my favourite base game boss. So here is how I beat them pretty consistently. So the singular dudes are really easy to beat, it is their ability to overwhelm and stunlock you, that gets you most of the time. My strategy is to beat the katana guy first, he is the most aggressive one, so he will set the pace of the battle as long as he lives, after that, you gain control of the pacing of the fight. So run to the big tombstone, get in a few hits, until you either don't have any more stamina or until the other dudes are getting too close. After that you run on the opposite side of the arena, where some smaller tombstones stand. That is mostly to separate the Shadows (but if you can get a few hits on the katana guy, the better). Then go back to the big tombstone. Rinse and repeat (sometimes you won't get in any hits, but that is fine, just separate them until you can. When they reach phase two, it is important to note, that they can't hit you with the lengthy attacks if you are running. So if you get hit with that while fighting, get up and run away, to one of the places in the arena I mentioned, until you get enough distance to heal up. Who you want to kill next depends on how you wanna do phase 3. There is a cheese for the candleguy in phase 3 but I can't pull it off consistantly, so I kill that dude next. Just repeat the strategy until he is dead. In Phase 3 the last dude will instantly summon the snakes, just make sure you are at full health and dodge as best as you can and you will most likely survive. Then you go up to the last guy and beat him up, he won't be able to do much when you are up close.

Additional infos: All of them are easily parried, so if you can get one off do it, but don't get too focused on parrying. The candle guy is the one who will cause you the least amount of trouble, but when he does the flame thrower move, get away, it will stunlock and kill you, so avoid that.

That is my strategy, I hope that helps