I'm not a bad player, but the vault final boss does need some adjustments. This fight is a bit random on where/who he targets his ground aoe and it can make it hard/impossible to dodge the knights.
The ground AoE is always placed on everyone and always happens at distinct points in his attack pattern. Nothing random about it. The only real frustrating thing to me about the attack is that the AoE circle is sometimes difficult to see; it doesn't contrast well with the floor in the room, and I think a lot of people get hit by it for that reason.

As far as dodging the knights goes, placement is everything. Allow yourself to be able to run to the south side of the room to dodge your plume, so that you have as much time as possible to squeeze between the knights.