“An Avicara near Mineral Springs may spawn behind portal requiring to rezone.”
That is what you are seeing in the picture: at the far side of the zone, a group of enemies can spawn outside the zone. Those tricks you’re thinking of to possible reach it? They don’t work. There is no outpost on that side of the zone, so to vanquish Tasca’s Demise you must cross the entire zone, and if that spawn is outside the world, you must leave through the far portal, come back, and keep re-zoning until those enemies spawn inside the zone.
This bug has presumably existed for years and absorbed hundreds of hours of time from players who stumbled upon it while trying to vanquish the zone without having read the wiki first. No one ever prioritized fixing a bug that made part of the content impossible without repeatedly restarting.