A sidequest that spawns at around roughly 15:00 in-game time in western-most part of the map, just south of the Decratom Haven.
It's a level 99 enemy but Aranea will do most of the work. Simply hide in the little weapons hut within the fighting zone constantly shooting from afar with rockets and sniper shots. You will have infinitely respawning ammo here so make the most of it.
When available, use Aranea's Highwind ability followd by as many crackshots as possible, then retreat back to the hut.
Very simple fight, just a slow grind to get through.