SMT IV you can actually level up pretty well solely through taling and fusing demons. The only issue being it's difficult af to talk and get anywhere without the demon deciding "I hate you now"
Yeah, in P2 their reactions to your contacts are fixed, but sometimes they ask questions, and their reactions to your answers are randomized... So even if you carefully do everything to form a contract, they may still randomly decide that they hate you and attack ;-;
That still makes me feel bad tho. Think of Nightmare: she is a Succubus in training who appears as a little kid. If you piss her off she strikes you with her normal attack... Which conists of clumsily flailing her arms around. AND YOU KILL HER IN RETURN. I feel like I am the demon here...
