Update, I got to the top 10 on the arena.
I had to change the location to the caves where there's no room to fly, but other AC units were also ridiculously easy afterwards.
As of now I am carrying two back weapons that allow extra ammo + the small shotgun.
Also I equipped some legs that give you more speed than the rest while sacrificing defense, but since the units at this point just annihilate 3k AP with one single hit, the extra movement speed is critical to survive.
A wide FC also has been crucial for times where I need to have the enemy on the corner of my peripheral vision, while also able to hit them.
This game has a lot of things to like, but I will not lie, some segments are real hairsplitters.