Roboquest

EDIT: Reload the game after your run is over while in the boot camp close to the main menu of the game than close the game then reboot it and upon clicking continue it should fix it(thats what fixed it for me)

Somehow glitched out on me I completed my guardian 4 run with the S average, and it gave me that achievement just not this one for some reason. kind of irritated i guess maybe if you kill the boss too fast it glitches out or something? hope this doesn't affect anyone else.

The character i felt best fit my playstyle was ranger. this due to the stealth ability being very very helpful if combined with shadow strike and a windmill rifle with shock on it. this was how i managed to melt a lot of the bosses. build into shock/crit and some defense if you can find it to help survivability. decrease the cooldown of the stealth as well mine was 9 seconds with crit taking 1 second off every 2 seconds. wild rush is a good pick as well with both of its sub upgrades being very helpful (extra auto crit chance and never needing to reload if you can hit your javelin throws).

to unlock guardian 4 you must beat a run on hard, then beat a run on guardian 1, then beat a run on guardian 2, then beat a run on guardian 3, than beat that to unlock guardian 4.