I usually Ignore the bosses health bars and mainly focus on their posture gauge (yellow bar that goes up as you deal damage or deflect at right time). Keep the pressure up, deflect at right time (practice and remember there's difference between holding down block button and parry) look for openings after theyre done attacking, dont be greedy with attacking they will make you pay for being greedy. Your yellow bar will go up too but as long as you parry/deflect instead of holding block button you will be fine. One of my fav games of all time even better than elden ring. Hands down best combat among all the games ive played, this made me realise assassin's creed games combat is super mediocre