In my most recent match of Nexus Blitz, we were losing pretty harshly. They'd taken both of our nexus turrets and our inhib was down at the time. So, when Bardle Royale began, super minions began to mass in front of our nexus and pretty quickly destroyed it without giving us a chance to fight back. This wouldn't be a problem normally, it just forces us to let go and let the obviously stronger team win.
But! This was going to be a decisive teamfight. Before we had a chance to fight though, the minions ended.
So, I suggest that the wall of fire executes or freezes minions and during the event, the Nexuses don't spawn minions.