I beat Bloodborne this week. It was my first "Souls"-style experience and I loved every minute, but without community help I really would've struggled on the boss fights. Here's some basic tips to help new players. If any of these words helps a single person beat a boss that's been driving them crazy, you're welcome.
NEEDLESS TO SAY: BOSS SPOILERS FOLLOW.
CLERIC BEAST
This motherfucker is probably one of the toughest bosses in the game purely because he's the first, and by this point you're probably still a mewling maggot of a hunter who took 25 minutes to individually pick off every single enemy at the bonfire section.
The biggest problem with the Cleric Beast was, I found, the fact that his tall stature made him difficult to get a handle on. If you get close enough to attack, you can't see his punches coming. I'd reccommend not using lock-on because the camera won't be your friend in this fight. Also, you'll gain 1 insight point purely for meeting him (and losing), which allows you to level up in The Hunter's Dream. If you struggle, get some blood echoes and level up your defenses.
FATHER GASCGOINE
Gazza is probably the game's hardest boss, at least according to the unofficial Reddit poll. He's the first "hunter" enemy you face, and his speed and capacity to interrupt you with gunfire are tricky.
Make sure you grab the Tiny Music Box item from the little girl, Viola, after you climb out of the bottom of the sewer section. Using this item stuns Gazza, and lets you get a good chunk of damage in on him. Don't use it in the first section of the fight - take your time to master parrying him with your gun, you'll need this skill later in the game. Once he reaches half health he turns monstrous, so now's the time to stun him with the Music Box and finish him off with relative ease.
Finally, don't go back and give the red brooch you get from him to Viola. You don't get any reward and if you keep it yourself you get a decent early-game blood gem.
BLOOD-STARVED BEAST
Oh boy, this one is so tough if you go in blind. It's one of the quickest bosses and inflicts slow poison on you very easily. Bring a full stock of antidotes to this fight. There's two things you can do to make BSB easy, though.
Firstly, recruit NPC help. You can find Alfred, a fellow hunter, in Cathedral Ward. He's upstairs in the building with the tomb, where you pull the lever to reveal the stairs down to Old Yharnam. If you're friendly to him, you can summon him by ringing the beckoning bell at the glowing circle at the bottom of the stairs leading to the BSB fight area.
Secondly, the BSB loves Pungent Blood Cocktails. They're fairly pricy early in the game, but it's worth grinding some echoes to give yourself as many as possible. Throw one of these away from you during the fight and it'll act as a distraction, making the beast ignore you entirely for a short time and leaving it open to powerful backstab attacks.
Combine the cocktails with the further damage and distraction Alfred provides and BSB will be easy. I bashed my head against this boss for nearly 15 tries. After discovering these two tricks, I beat him without taking a single hitpoint of damage.
VICAR AMELIA
Personally, this was the hardest boss in the entire game. I just couldn't cope with Amelia's size and her massive number of different attacks. She's very powerful, capable of one-shotting you at your current level, so make sure you level up until you feel like you can cope with taking a blow from her.
Her wavy fur also messes with your depth perception. I often thought I was within range, only to whiff a charge attack completely because her ragged coat makes her appear closer than she is.
She's weak to fire, so bring fire paper to imbue your weapon with flame. Stick close to her, and try and dodge under her arms when she swipes. There's a few frames of invincibility in your dodge move, so don't get disheartened by repeated attempts and use them to try and get a feel for the right time to move and attack. Don't get greedy on the offensive because it's easy for her to catch you out, and if she sits up and starts praying she's about to heal herself - start pummeling her as much as you can to cancel it out. This fight was an endurance battle for me. Just stay patient and you'll eventually come out on top.
WITCH OF HEMWICK
This boss is invisible at the start of the fight, and she makes at least one copy of herself that you have to defeat. Try and ignore the creatures she summons as much as possible, as they'll just summon more if you try and focus on killing them off first.

Stalk the edge of the room and keep an eye out for a glowing red light. Once that appears, make a beeline for it and use a charged R2 backstab on the witch as she fades into view. This will open her up for an R1 visceral attack for great damage.
Don't get caught up fighting summoned minions and this fight won't be too difficult.
SHADOW OF YHARNAM
Speed is of the essence in this fight. Use your fastest weapon (for me it was the one-handed form of Ludwig's Holy Blade), and keep on your toes, constantly weaving and moving to try and draw your three foes into short one-on-one fights.
Duck behind the large headstone in the arena at the start of the fight to hide from the projectile Shadow. The melee Shadows are easily stun-locked with a quick weapon, so pick your moment to attack and take them out. After a set amount of time/damage, the Shadows get new attacks. Watch out for the mage's second form in which huge snakes are summoned that can catch you off guard.
Be prepared to run this fight a few times. Figure out which Shadow you feel is best to pick off first - some prefer to get rid of the mage's irritating fireballs, I preferred to get the in-your-face sword Shadows out of the way so I could focus. Stick to your preferred strategy and keep the tempo of your attacks high.
ROM, THE VACUOUS SPIDER
First of all, ignore the small spiders throughout. Keep on the move, don't use lock-on and don't try and get any more than three or four attacks in before preparing to dodge.
Rom's moveset consists mostly of predictable bellyflops, but he has two things to watch for. If he starts to glow reddish he's about to use a powerful area-of-effect attack, so get some distance between you. The attack that causes most players issues is his missiles. If you see him rear up high when you're far away, pay attention. His icy missiles will hit the ground
at the moment his belly hits the ground and not a moment later. If you roll twice to the side at the moment he hits the ground, the missiles will miss you completely.
Other than these two sneaky attacks, he's fairly easy. Make sure you keep your health fully topped up throughout this fight, as it's easy for a spider to finish you off with a cheeky swipe after getting caught with one of Rom's powerful bellyflops.
THE ONE REBORN
This is the most boring boss fight in the game. Get yourself some bolt paper for extra damage and stand at The One's side, by his tiny little spider legs. Here, his big sweeps are easily dodged, and any damage you take from the flailing little legs will be mostly negated by the damage you deal with your own weapon.
The heavy version of Ludwig's finished this boss off in record time for me, and The One is a slow mover so staying clear of his attacks should be easy.
DARKBEAST PAARL
I have to admit, I got quite lucky with this one. I stumbled upon it by accident whilst exploring Yahar'Gul when I had one blood vial to my name, but managed to beat it on the first attempt with a sliver of health remaining.

Paarl is quick, but not exceptionally powerful. He's a lightning-infused beast that doesn't have a massive moveset. You should be getting adept at dodging by now, so keep your cool and dodge underneath him when he swipes at you. Your most powerful weapon should do good damage to Paarl. Maintain concentration and this fight will be relatively routine.
AMYGDALA
Amygdala's head is the weak point, so find which of your favoured weapons has an attack with high vertical reach. Ludwig's two-handed L2 attack worked for me, and I've heard the Cane Whip is effective too.
Amygdala is fairly routine until the last portion of her fight, when she gets a massive boost to her range. Make sure you have a lengthy health bar by this fight, or you'll get taken out very easily. Focus on the head when you can, and manage your distance to her in the second portion of the fight so you don't get caught in medium range where she'll be able to hit you, but not vice-versa.
CELESTIAL EMISSARY
When you walk into the mob of little aliens, pick a long, sweeping weapon such as the hunter's axe. Determine which alien is the real boss, then damage it until it transforms.
Once this happens, run away upstairs, back towards the nightmare portal. The Emissary won't follow you all the way up there, so you're free to hit-and-run until it's dead, or just lob molotovs at it until the remainder of its health bar has gone.
EBRIETAS, DAUGHTER OF THE COSMOS
This applies to most bosses in the game, but more than any other this one:
get behind it. Ebrietas hits fast and
very hard, and she has a huge variety of attacks. None of them can hit you if you manage to get between her back tentacles, leaving her open to massive damage, especially if you use some bolt paper.
Just keep rolling round to her back if you find yourself face-to-face and she won't be able to hit you. Additionally, your health bar will continuously deplete in the latter stages of this fight, so bring blood vials and keep one eye on it so you don't find yourself caught short.
MICOLASH, HOST OF THE NIGHTMARE
Micolash himself is a pushover. He only has one or two attacks, none of which are powerful, but he will try to confuse you by fleeing into a maze twice. When chasing him, try and keep track of your steps so you don't get disoriented, and when he locks himself up in the second phase of the fight, go upstairs and you'll be able to drop down into his cage. Otherwise, the easiest prey this game has to offer. Be sure to scour his maze after his death, for it contains plenty of valuable booty.
MARTYR LOGARIUS
For the first half of his health bar, try and keep close to him. He's mostly a ranged fighter to begin with, launching powerful missiles at you to zone you out, but his melee attacks are easily dodged or parried. If he stabs a sword into the ground, lock onto it and fire a quicksilver bullet at it to save yourself from an irritating barrage of knives.
In the second half he will buff himself and be immune to parries or interruption, and will become far more aggressive and melee-focused. When this occurs, attack with care and only try to get two or three hits in before dodging away. Use the roof spires to give yourself breathing room if you need it, and be patient in your approach.
MERGO'S WET NURSE
An easy fight if you stay behind her. Mergo's Wet Nurse doesn't have many attacks, and her slashes are predictable. When she attempts to hit you, get round to her back and you'll be safe from attack. Her other notable attack locks her into slow, forward movement. This is pretty much a freebie to get to her vulnerable side and go nuts.
Halfway through the fight, she'll turn the arena dark and summon a clone. Don't try and engage her at this point. Run in circles around the edge of the arena, rolling repeatedly if you see her claws draw near. After a short while this phase will pass and she will revert back to her earlier state.
GEHRMAN, THE FIRST HUNTER
This feels a bit like a final exam. Everything you've learned facing the hunter-type enemies in the game so far has to be applied here. Gehrman isn't particularly tough if you're a strong level (I found him relatively easy at level 99), but like all bosses he takes a run or two to learn his attack patterns.
Luckily, he doesn't have that many different attacks to catch you by surprise. At the start of the fight he wields a scythe and his attacks will suck you in close for more hits, so be patient, try and keep to the middle distance and pick away at his health whenever he telegraphs an attack.
At about 50% health, he'll switch to a sword and blunderbuss. This form is easier than the first: try to stay in his face, and if you're trying to close him down, always dodge towards him if he tries shooting at you.
In his final form he'll have a more powerful gun that, if it hits you, will leave you open to Gehrman's own visceral attack that does huge amounts of damage and will probably one-hit kill any players under level 85. Employ the same strategy: dodge towards him, make sure you're a safe distance away before healing up.
THE MOON PRESENCE
You're on your own.