I got the Harem ending, wasn't that difficult to be honest, if you have trouble getting love or lust try spamming the kiss option, if you can only use it once due to stamina, it wont be used at all so you will essentially have infinite stamina while you spam kisses. And for the final fight with Agent 3, use Marie, she is the best one out of all three of them when it comes to fighting her, Callie is too slow, and Captain 3 doesn't have enough range. Though it isn't impossible, it is very difficult and really isn't worth it, though each of the characters has unique dialogue depending on their love/corruption status for each opponent.