This game is very complicated and just leveling up your Sync Pairs will not help them max out their possible and end up being as helpful as they can be, although this is the main point you should be concentrating on. Apart from leveling up, the most essential thing you should do to increase the total strength of your Sync Pairs is unlock new capabilities/ moves. You will require Training Machines of different rarities and, in addition to Aid Ade bottles--however early on, things will be much easier and requirements low. Since investing in Trainers and Pokemon you will not use is just a waste of resources, just focus on the sets you're in fact preparing to use a lot. If absolutely nothing else, just look at the rarest Sync Pairs you have (more star numbers) and focus on those.