As you play the game, you will acquire gems which can be added to your weapons. Each weapon can hold 3 gems. One set of gems is elemental, related to the 4 in the game: Acid, Ice, Fire and Electric. Each requires to get either 100 or 200 kills per element. Equip a gem focusing on the element into your go-to weapons. To enhance this there is also a gem which adds bonus damage to elemental attacks. Further more there are various skill cards you can acquire and which further add more damage done. There are plenty of sides missions and bounty board missions which give you kills to make this attainable.