I found the Valk fights are very repetitive...
They're not difficult, they're sponges that take little damage and deal out a lot...The way they absorb damage is the problem, it prolongs the battle (each took me about 15-20 minutes on challenging) thus increasing the odds you get caught slipping...Its hard to be perfect for 15-20 minutes,get caught slipping three times with one of them and you're dead...
That's how I took L's, would start off great and then get caught twice when the health-bar was half way depleted...
That's not challenging design IMO, that lousy design...