He finally spawned for me after 183 placeholders. There are 6 angry Arcrons in total:
3 static mobs - by the pillar near Gorowyn, by the lava river, and by the spires/dragon orb thingy to the south east. It spawned twice in row, by the first two spots mentioned
3 roamers - two that are mostly in the ruins shown on the maps: one that roams near the quest npc, and one that roams from south west and into the pillars area. The third roamer tends to be a little further east and moves out into the open field.