Try using our Smart Pet Search
and selecting "Family is (your pet type here)*", and then selecting what pet family you want to have a strong ability for.
Example: Family is - Aquatic, Strong Against - Flying. This yields 4 results, each an aquatic pet with at least one ability (Magic-type) that's strong against Flying.
Keep in mind that the search may come back with pets that only have one ability that matches the "Strong Against" filter. So the results might not always have pets that are ideal counters for other pets.