Boss ritual last rank

i don't know GGG why do this and i don't even understand too for what. i can finish boss more 50% hp then boss should next phase but i don't know what happen ? or what a boss use skill ? ONE HIT DIE . i lost my ritual key like a WTF!??

Is it fair to the players? Some people take all days maybe many day to find the shit ritual key drop rate so many many low maybe some people buy it 6-7 Divine for face to ONE HIT FORM BOSS!. WOW! That's so very very very fair!!!.

I just feeling like What the hell man. farm for face just one hit. huhhh!?