There are several ways to make pets stronger:

• Pack: The more pets of an element have been explored and found, the more comfortable they feel. Per pet in the pack, their attributes automatically increase by 5%.

• Food: On quests (without an item reward displayed) fruits for pets can be found. In pet and habitat fights you get fruits as a reward.
  Each element has its own fruit and increases the pets level which is assigned to the element and, thus, its attributes when fed. A pet can be fed up to three times per day. The maximum level for pets is 200 (100 if not all habitats of all elements have been explored yet!). To feed a pet, drag and drop the respective fruit from the fruit basket onto the pet to be fed.

• Level: If a pet reaches level 200, the attributes of all pets of the element increase by 5%. You also get 5% if you have two pets between levels 100 and 199.