Just got to Aldwynn and these knights in golden armor (think they're called Drake Knights) are just wrecking me. Harder than any of the bosses so far I think; they just attack with seemingly no stamina restrictions, fire lightning bolts which leave lightning balls, and on top of all that, they bloody HEAL THEMSELVES. How do I deal with them?

https://www.youtube.com/watch?v=JnXZpsYJMAg

Watched the video, seems like he's a using a trainer or something, since it seems that he gets no damage after taking hits. My character is level 72, and I faced 3 of those foes, and only managed to kill 1. The parrying system in this game seems really weird.

Crystal Bombs - they're available from the merchant in the sanctuary. Buy ~10 per knight. Stand just outside of aggro range and spam bombs to stun-lock the knight. If the knight jumps into the air when almost dead, finish the fight with a jump attack.

DPS. With enough damages, you will stunlock them every two slashes. See the big fellows after the Soul of the Phoenix, with there Crystal Greathammer? I came back there in the end of the game, and each strike of me stunlock them (Sentinel Longsword with 40 (base) + 21 (equipment) Strength). I don't really know if this is related to damage output, but I don't see any other reasons for this stunlock stuff.
The Heroic Soul of the Phoenix was stunlocked every 2 hits.

STUNLOCK POWAAAAH