From personal experience, mostly know the enemies max hit, and what they use, mostly comes up to research for me. If I PK on an obby pure, I know specific strength levels max hits, depending if they use prayer or not, so I rarely die by knowing the max hit and then you could just do easy bneck risk fights. For other pures like gmaulers I guess the spec for gmaul is mosly RNG luck.. So I guess practise will make you consistent and knowledgeable.