Thanks for your answers guys,
i saw a video with a single man soloing Hydra boss with like 800 arrows or more, he used glider to switch between high cliff. About the caves where artefact key should be used to get Power Stone, i cant find any maps/loca except streamers video and those caves are full of high lvl monsters !
Power Stone Islands :
C6 = The Blue Stone
D12 = The Turquoise Stone
F7 = The Green Stone
G3 = The Yellow Stone
H6 = The Orange Stone
M12 = The Red Stone
O7 = The White Stone
O14 = The Purple Stone