That is the case with Dark Souls bosses typically. If you know the counters and are reasonably statted you should be able to melt their HP bar. The difficulty is in learning the counters.
For instance, the boss Yora is stuck on - it took me a similar amount of time to be able to beat him, and whenever I come back to him there is a period of "re-learning" how to fight him. For a pro player or a speedrunner though? It's about 30 seconds of bullying the poor guy, and it's only that long because he has different phases.