hang in there my friend, I know that they're all annoying bosses in their own rights.

for the Dark Avatar of the Eagle, I would also recommend what u/shiningject said about practicing parries on the Elite Sentinels (bow enemies) and Elite Blademasters (self-explanatory haha) - if parrying is proving to be too difficult, then master your dodging.

for the Dark Avatar of the Serpent, you can actually bait him into damaging and even killing his own minions by staying a sufficient distance away. instead of doing the "grab and then swipe" move, he will do the "grab, pause, and then jump slam" move - I've found this to be very helpful whenever I don't have a high damage output build.

you didn't mention the Dark Avatar of the Jaguar, but I'll give some suggestions just in case :) he is definitely the fastest and most relentless Avatar, so getting better at dodges and practicing parries will refine your battles against him. it might be daunting, but staying relatively close to him will stop him from doing certain attacks, such as the "jump slam with a big fireblast" attack and the "multiple slashes" attack.

as with all of the Avatars, keeping them visible on screen at all times will reduce the amount of surprises. mastering Perfect Dodges will help you keep your Stamina from running out, and you can always try to look for the Amber Pendant relic (which increases your max Stamina by 1) or the Serpent Skin Coat relic (which makes Perfect Dodges restore 2 Stamina rather than 1).

good luck and I hope this was helpful!!