If Mori wants to learn how to beat the bosses in Fromsoft games, and since she's probably lurking, it's more or less of a mindset thing:
With how experienced the developers are, they would not put something in the game that the player, themselves, would not be able to beat (story sequences notwithstanding).
By that, I mean Miyazaki himself doesn't play action games (or didn't prior to Souls), so if he could beat the bosses, players would be able to perform leaps and bounds ahead of him with enough work.
That, and each major boss and enemy telegraphs their movements, meaning that fighting them will eventually become muscle memory, which is by design.
React quickly, react predictably, but what is most important is reacting accurately. Knowing what will happen, seconds before it happens, will then boil down to hand-eye co-ordination, which also increases naturally while playing games.
With that in mind, you can't run before you can walk, so start out with learning how to utilize the base toolkit a character has before any additional equipment is added. Take down basic enemies until you can do so with minimal health lost, learn to dodge, learn to parry, get a feel of the iframes at your equipment weight. Unironically, you'll eventually learn to 'feel' each moment-to-moment gameplay beat if you're concentrated enough, at which point you'll be gunning for the Tree Sentinel on a fresh character without enhanced gear so that you can get an advantage over Margit, who you'll also clear in one go because he's just Margit, there's bigger kills further down the road to worry about.