Fusing Personas is essential to getting a huge advantage over shadows, as they greatly affect the flow of battle for your team. In this guide, I will explain all the methods on how to fuse higher level Personas in P3 Reload.
How to Merge the BEST Personas in Persona 3 Reload (Tutorial)
If you want to sort of cheese Tartarus in Persona 3 Reload, you can fuse higher-level Personas at fairly low(er) levels. Here are a few ways to do it.
One of the important aspects of playing a Persona game is Social Links. If you're not familiar with how S-Links work, maxing them out allows you to fuse an Arcana's Ultimate Persona (for example, Cybele for the Lovers Arcana).
S-Links also provide an EXP burst from Persona fusions under their respective Arcana. In the picture above, I fused Dionysus (Moon Arcana). I gained significant EXP from my Rank 8 Moon S-Link boosting Dionysus to Level 53 (originally 49).