Fierce is, in my opinion, the second best DPS trait. It comes only behind Brutal. The main difference between Brutal and Fierce is that Brutal applies directly to strength so it affects shields and healing as well as damage traits. With Fierce, you want to focus on characters who have multiple DAMAGE based abilities. This is a very popular trait that you will often see paired with Brutal to create a HUGE damage boost for your champions.
Damage =
(attacker strenght – target shield – target fortitude) x multiplier, critical/fierce
From this you can see, what fierce without enough strenght can deal lesser totall damage e.g. against heavy tanks. Without additional buffs for strenght from any teammate (like Hulking) can be terrible to use it