To start with, there is minimal indication that you're about to fight a boss, since there is no health fountain or magic fountain before you enter his room and the area right before it has you fighting like 3 or so fiery skeleton knights, which are pretty rough. So I basically walked into the boss room with half health and no light magic.
Well no problem I'll just build focus by dodging and counter attacking everything just like I've done for every other boss and gain some light magic to heal myself. EXCEPT NO THIS BOSS IS ALMOST IMPOSSIBLE TO GET A COMBO CHAIN OFF SINCE HIS WEAK SPOTS ALL REQUIRE YOU TO BE AIRBORNE TO HIT! Gabriel Belmont has almost no functional control in the air besides continuing to attack and a double jump, he has a ground slam attack but it has long wind up. To further compound this issue the bosses ground stomps hit you while you're airborne, and his recovery time between attacks is extremely quick. Essentially you're fighting a hyper aggressive monster who is only possible to hit when you're at your least mobile and most vulnerable since you can't block any of his attacks period, least of all while airborne. For nearly three hours I slaved away learning patterns.
It's a multi phase boss fight and each phase he gets faster and more aggressive, however thankfully there are check points. Which means I get to respawn with half my health each time I make it to the next phase, his attacks hit hard too so you'll probably die shortly after each check point. You can only take ~5 hits depending on what hits you before you have to reload.
Basically what makes this boss awful is that you don't have to apply ANY actual game knowledge to find a winning strategy, the focus regeneration system becomes useless because actually combing him is a waste of time and effort, you don't have the window to perform any good air combos and you cannot hit him with ground attacks period.
Secondly, your subweapons? All of them useless, knives don't track well enough to hit him, holy water is ground based damage, fairies do nothing, and the huge damage crystal doesn't hurt bosses.
Thirdly, his attacks aren't intuitive to dodge. All his ground stomp attacks hit you while in the air, and his dive bomb attack is only dodgable if you have a running start, so if you are mid attack or mid jump while he begins it you won't have enough time to active cyclone boots therefore you get hit.
Fourth, it is unusually and unnecessarily punishing. If you get caught mid air in a combo and he starts revving up either his spike thrust into the ground or and ground stomp attack you're hosed. Your best course of action is just own up to your mistake and keep combing as long as you can because even if you stop now you're still going to get hit. Some of his attacks are only avoidable if you're ready to dodge them 2 seconds before they start to happen.
I guess you could say that the boss is extremely demanding, which it is however it's not a fun challenge. You don't feel good when you finish it, because your strategy for beating this boss will actually just be you waiting for him to do his double sword attack so you can get 4-5 hits in, any more and then you get hit.