After they buffed the sticky bomb damage the most common method to kill every boss is using it. For just 64 $ you reduce the boss health by 3/4. After that you just have to wait until the frenzy mode is over and u can finish them with just 2-3 melee hits. You can shoot them aswell.
Because this variant is so much faster there is no more need to care about the noise.
This wont work with the Assassin anymore.

Killing the Spider is super simple if she is in any room without a high roof. You only need a strong meele weapon. Just look for a sledgehammer and attack her with charged attacks. After some charged hits she should be dead and you only need about 30-60 seconds to kill her.
Take care of the frenzy mode. After she lost 1/4 of her health she start screaming, gets reduced damage and instantly starts attacking. Until she calm down after some seconds. Just leave the boss location for some seconds and wait until she screams again.(You can also throw lanterns on her, which deals good damage but isn't as stealth as just hitting her with meele weapons.)

If u want to kill the Butcher fast and stealth you only need 48$ for an concertina bomb. Just throw it into a small passage and lure him into the barbed wire by making him agressive. You can do this with a silenced weapon or just with your presence. Avoid getting hit if you are close to the barbed wire because the Butcher can break it if he is too close to it.(They nerfed the damage form this method. It still works but it deals much less damage then before.)
Like the both other bosses you can hit him with a sledgehammer. Just hit and run and avoid his fire attack.
Like the other bosses he also have a frenzy mode after loosing 1/4 of his health. He will start to chase you with the same speed as a sprinting hunter and constantly attack with his hook. Leave the location and wait some second until he clams down.

Killing the Assasin is very easy and basically like killing the Spider. Just search for a hammer or an axe or use any other meele weapon and wait that he chases you. Now you just have to wait that he materializes so you can hit him, but take care that you run if he dashes to you. If he hits you while he dashes you will bleed. Do this a few times and he get stuned for some seconds and you can give him hell. At long last a silenced weapon works also but it will be much faster with a meele weapon.
Take care of his frenzy mode which will start when he lose 1/4 of his health. He will spawn clones which deals some damage and blinds you.
What also deals a good amount of damage is to set him on fire with lanterns for example. He is also weak against poison.

Yaaay finally they released a new boss. Scrapbeak comes with a lot of crows in his compound, so it's very easy to spot him. He should be one of the easiest bosses in this game cause he is weak to poison, quite slow and his only specail attack is throwing barbed wire.
What makes him special is that this greedy magpie collected every single item around him. But you can beat everything out of him. If u attack him with melee weapons (piercing damage is the best option) or guns he drops his loot every time he gets enraged. The important thing is that you shouldn't burn him or throw explosives if you want the loot, because otherwise it gets destroyed. But at the end the stress isn't worth it. Just use a sticky bomb like on the other bosses and you save a lot of time.

With the latest update they added silencers for the Winfield and the Sparks. But the silenced Winfield is the only gun with enough dps with which you can kill the bosses. It could be especially used for killing the spider in rooms with a high ceiling where you can't reach it with the sledgehammer. At last you get just a small advantage with buying a silenced Winfield for the price of loosing some firepower. After they added levering perk it made the winfield much more valuable.

If you want to be much faster you can just throw 1 big dynamite bundle right next to them which should kill both bosses instantly if thrown right.

Thanks for reading, good luck and enjoy the hunt! :)