Best build for power charge stacking?

Looking at poe ninja, it seems like 70% of players that have 15+ PC are playing Slayer, and only 14% playing Occultist, but why is that if Occultist has PC bonuses in her ascendancy? It seems like people playing Slayers are all going for all stacks, rather than just PC. But I'd want to know what would be the mirror tier build that focuses only on PC?