On PS4, I started the Boethiah’s Calling quest at level 27 and killed all of the cultists except one. This one, in particular, won’t take damage and knocks my follower every time I try to go to the pillar. I gave up and did other quests. I’m now level 43 and the cultist still doesn’t take any damage.
As per the UESP Wiki article on Boethiah's Calling:
If you kill the priestess before talking to Boethiah, one or two of the current fighters may become unkillable.
You'd have to load a save where the priestess hasn't been killed yet. And then don't kill her before talking to Boethiah.
If playing on the PC, you can use console commands to fix the issue:
Open the console and for each fighter that's unkillable, click on them and then type
kill. The quest will proceed as normal once all the fighters are dead. If that does not work, open the console and type
damageactorvalue health 500.