You ready to be pissed at yourself? The Ancient Dragon is beatable by anyone with a half assed mele attack. Step one: Run in between the toes of one of his back feet and chop. Step two: When he tried to stomp you, roll to the other foot and repeat step one. It is literally that easy, because once you are between his back toes, all he does is the one foot stomp attack. You could probably beat this fight with a SL1 guy with a rusty dagger if you didn't get bored. This only works if you are solo in the fight, as if he targets another character he may do the air breath thing and that is usually an instant wipe.
Vendrick is a lot more tricky, but there are a couple of things you are likely doing wrong. First, his hitboxes are kind of fucked up in certain ways. When he does the big swipe to his left, the end of the swing will not actually hit you if you are close in on his left side, even though graphics wise it appears the sword is hitting you. So, by keeping to his left side, you are largely safe most of the fight, except for when he resets. When he does reset, you need to move far away from in in front of him to get him to use his leaping attack. If you do that, you can roll under it and be directly behind him again (safest place to be). His most dangerous attacks are the circular swipes, which can be dodged by rolling into them. The second thing to do is to never get greedy in this fight, because he can reset in some really weird and random ways that will get you out of position, so you need to be ready to roll the correct way at the drop of a hat. Double hand your weapon, strong chop him once only when behind him, and maintain your positioning on him. At least, that is how I finally beat him. My character at the time was a dedicated caster swinging a Raw BK Halberd +5 and with all five giant souls it took me about 3 minutes to down him. Once I figured out that his hitboxes were messed up, visually, it took me like two tries to drop him.
The Umbral stuff was probably the hardest crap in this game to beat, in terms of execution. Having to take on what amounts to 3-5 cheesed out characters in shitty environments was bad enough, but if you fuck up at all, you are out an Effigy. In theory, you could just bum rush the torches and light them, but the little thief guys have sync kills and can stun lock you besides. I basically abused the fuck out of Dark Vortexes to clear them out. The actual boss is kind of a pussy if you are doing the faith lightning thing (I one shotted him without having any idea what he was like), but looks like he would be a serious pain in the ass to a dedicated mele guy, since his spells run the full range of resistances. The clearing to get to him is 100x harder than he is, though. Imagine trying to fight off 3-4 invaders in the Gutter and you have a pretty good idea what you are in for. I cannot comprehend how a dedicated mele guy tackles those areas.
Anyhow, I tidied up my first character (beat all bosses and maxed the covenants I cared about) and decided I would rather try a mele centric build the second time through, rather than go on to NG+. Game plays completely differently as a two hand specialist and armor actually means something for at least the first half of the game. Got all the way to Smelter on day one, but that fight is making me rage as a mele. Still enjoying doing things differently and the game is a lot more efficient when you know where everything is, plus you get to see bosses use new moves that you never experience as a ranged person. I wonder if I can make it through the game without my Dark Vortex crutch. For the record, I probably failed on RRA more than any single boss in this game.
For the person who asked about Hexes, there is a limited group of useful ones out there. Dark Orb is a solid damaging alternative to other spells in its stat range early on, the poison cloud is like having cheat codes in certain areas, and Vortex is a game ender. The higher end Hexes are powerful, but either eat up way too many slots or cost souls to use. Basically, I would start the game out as a Sorcery caster until I hit about 24, then start building faith. Attunement is a must to hit 30, even if you are slotting the gear to increase spell slots, because the Vortex Spell eats two slots per copy. Any chime crafted up to max and packing Dark Enchantment is going to work fine. The couple of staffs that are hex centric are not really worth it, since the Shrine Staff is almost as good and late game you will rarely opt for Dark Orb over the faith lightning spells, anyhow.
Once your fighting stats are at the bare minimum for whatever weapon you want to main and your attunement is at 30, all remaining points should be split between Int and Faith, with Faith taking a little more importance late game (everything is weak to lightning after Looking Glass). I typically ran around with four to five Lightning spells, Vortex, and the damage immunity spell late game, slotting in the poison cloud for specific farming areas only. Early on, though, Dark Orb is the go to spell and you will rarely slot a chime because a lot of the early Chime Hexes suck, frankly. Once you hit late game and can put up the damage immunity hex to deal with burst damage griefers like Jin, you will see the payoff in PvP. I found lightning to be the most reliable spell in PvP, since it is hard to dodge and does lots of damage (Sunbro Ring and Ring of Faith help a lot). Keep your cast speed at 150 or more and you can roll cast guys fairly reliably when you have to.
Of course my perspective is one of someone who does PvP only when forced. People loved having me along for coop boss fights and farms since casters are so rare and most don't have their damage optimized (Faith Lightning is the best for longer fights, stacked Geysers for DPS fests).