Play the psp remake if you're not in it to laugh at the worst 90s localisation has to offer. Past that, nuclear damage (megido line) is very strong, not much resists it, they always hit every enemy and they do more damage than spells of other types of the same tier. Any persona with one will carry you for a long time (Like Vidolfnir and Gozu-Tennoh). Always pack a healing spell on someone (well duh), sp is very easy to come by so even one character with dia will keep you going forever, since everyone gets a full hp/sp heal on level up. When you hit MAX rank on a persona, if you delete it at the velvet room you'll get an item, which can be anything from garbage (stat+1 items) to super ownage (awesome equipment like that charm spear with 5 hits, or tablets for fusing ridiculous skills onto other personas). It's usually worth it, since theres only so far you can take a low level, low stat persona before it starts becoming a liability anyway.
Talking to the demons might seem pretty obtuse, but it's not really that complicated. All you really need to know to start is that if you want to get a demon's spell card for fusion, you have to fill the Eager bar, and the response that will fill it depends on their personality traits (so Cheerful demons like stuff like Dance, Sing, Invite, etc). However, the closer the timer is to a full moon, the more likely it is that they'll take it badly regardless.
Anyway, despite the weirdness (why are there like 18 attack types? Why are some resists/weaknesses to them never shown in the analyze screen?) the game is rather easy for the most part, especially the main route, so you probably won't have much trouble. Best advice of all, hit the start button (or it might have been select) in battles to speed up the animations because jesus christ will you see them a lot
