My strategy that saw me comfortably through most of the game was to focus my MC as a physical build character and give them Death, Wind and Ice Dracostrike attacks, physical attacks that give bonus Crit damage (Bestial Bite, Heavy Blow, etc) and for a while I had Life Drain. I also used Belphegor’s Essence as the basis for MC’s elemental affinity and then used other essences to counteract the subsequent natural weaknesses.
For the rest of my team, throughout the game I generally focused on holding onto Resist / Null skills even if a demon was already resistant, and holding onto elemental skills that weren’t covered by MC’s elements - Fire, Elec, Light. I generally held onto single-target spells unless it was Light / Dark element as I often found that you can pretty much insta-win a lot of battles in the second and third (and even parts of the fourth) areas by using Mahama / Mamudo and light / dark shards.
Other than that I knew that my end-game party would broadly consist of one physical powerhouse, one magic attacker / buffer, and one magic attacker / healer.
When it comes to general progression, I found that you don’t necessarily have to have a well-crafted team to explore, and that potential fusion choices will generally match what you need to resist bosses once you reach them. I’ve found it remarkably forgiving, much more so than Nocturne.
I guess the tl;dr version is “don’t worry too much about it”.
