The exam was that you have to have 30 point in strength and finally became a demon slayer. How to get lust, was to able to get the character trust up so high and choose their event, like Shinobu for examples on relaxing with Her on the roof. I got only sees first before turn into a kiss and a ahem! Scene and the lust went up. Or, you asked for a favor to them, like a kiss on the cheek. That's easier.