Perks are a new addition to horde in Gears 5. Each character has their own specific perks which can be acquired and upgraded throughout a horde match. Del does not have any perks because he is an engineer, so you will need to play any other character.
These are in the lower right hand part of the screen and give you certain abilities such as ammo regen and bonus critical damage.
You are only able to attain and upgrade perks in between rounds and not during. For this achievement you need to acquire three out of the four perks for your character and then get a kill.
The first level of each perk costs 1000 power. Keep playing until you have saved up 3000 power and then after finishing a round press
to pull up the perk menu and purchase the first level of three different ones. After you have three separate perks start the next wave and kill any enemy and the achievement should pop.