Confluxes, Atheon and Gatekeepers the whole group has to come through. But it seems most of the time the Confluxes end up being the toughest overall. They are all doable and none of them too difficult but if I was to pick the most difficult section it would be the confluxes due to the fact it takes the longest to redo. Die on Atheon no big deal he only takes a couple teleports to beat. Die on Gatekeepers, no big deal only takes a minute once you get it right. But with confluxes you have to redo the whole three stages which takes a bit of time.