I find it strange how the game suddenly hit a difficulty spike for me for no apparent reason... I did the first three temples (Wind then Water then Fire) with no issues whatsoever even on the bosses, then I went to the desert area and did the Lightning Temple with 13 hearts and for some reason the temple boss kills me in one hit no matter what attacks touch me (the sand beam and just trampling me both one-shot me, the adds don't).
Seems a bit extreme. I eventually got it but even in the dephths some of the boss-like enemies take like 10-12 hearts away when they attack you. Same with Gleeoks on the surface.