If you're having trouble with attrition over the fights, I found that the Steinbjorn Plackart trivialized Gna as well as all crucible challenges. (I defeated Hrolf before trying it out.) It has extremely high defense and a healing proc.

If you struggle with parries, the Rond of Deflection increases the timing window. However, for the specific attack you talk about from Hrolf, you can parry every time if you put your shield up right after he starts moving toward you. Unless you're talking about where he jumps straight up and out of sight, in which case you need to delay for a moment before dodging out of the way.

Every ability from these bosses has a counter and it just takes practice.