In the missions and events, experience points are also award when successful. Titans should be the same. When a Titan is successfully defeated, experience points should also be rewarded. It could be as simple as a % of the damage a player does on a Titan. Using the star rating of the Titan as the % would be a simple method. 6 star Titan means 6% of the damage the player does is the experience gained. 1000 damage would reward 60 experience.
5 star = 5%, 6 star = 6%, 7 star = 7%, etc.
From a programming perspective, it would be an easy method to code by using the star level as the percentage.