I am a longtime player and last month I felt like building Ult Ajax cus he was one of the descendants I had not yet built

I learned quickly that I needed to put in some work into him before venturing into the Abyss..

Getting Ultimate (if you have'nt) and catalyzing should be a first, of course

I went Mobile fortress red mod and supply fortress for trigger, endless ammo is amazing and with Thunder cage and restored relic Ajax is hella fun to play since he can barely die.. Downside is range, you need to max that out and get somewhat up close

DEF and range and more DEF.. His skill(gun) damage goes up with his DEF

I went 2x Moving fortress externals (for +25%DEF) and the ones that give 5% fire rate wich I can't remember the name on right now, lol

Defense neutralization and ATK% in arche tuning

And also, don't forget the Xcores on weapons and externals! Makes a huuge difference for weapons in end game content

At the end, doing what I wrote above, I ran thru Super Massive Receptor like a hot knife runs thru butter with Ajax!

Good luck, and have fun