-( Story related, cannot be missed )-
The fight with Harriet, and the ensuing boss fight after, aren't as hard as the previous fights. That being said, considering you get them one after another, there isn't a moment to rest or recover, so be prepared as you're heading after Harriet as it is a point of no return. The fight with Harriet is much like the one with Doris minus the scimitar and more like the Ichors. She will fall easily, but the real fight is what awaits you on the other side. The Red Queen's Avatar is much harder than Harriet and will be forced onto you shortly after finishing the first.
The Red Queen is a basic combo creator, doing a simple one-two or one-two-three punch. What makes her hard is the fact that you just finished the previous fight, so be sure to bring some vials of blood or health serum. She can also cause a blood implosion from a distance, sort of like Abyss. She will summon Skals made entirely of blood that will try to overwhelm you. Keep your distance away from both the Red Queen and the Skals and they will dissolve with time. After some time, she will form scythes as the Skals surround you, and then try to dash at you from above. From this point, her attacks will hit harder and swipe further. If you continue to dodge and attacking when she's cleared you, you shouldn't have too much trouble.