[X] Come to a diplomatic agreement. All Caster wants to do is... well, you don't really know what she wants to do, but there's gotta be a better way to resolve this, right?
[X] Accept Caster's terms and offer. You can't pass up True Immortality, and you have Saber and Assassin to help you. Even the Magus Killer can't stand up to two Servants, right? Right?
[X] Counter-offer. You won't accept the current agreement, but you can work with her in a different way. Preferably, a safer way. (Write in what you'll be allowed to do.)
[X] Refuse Caster's terms and offer. Many people stronger than you have tried to fight the Magus Killer; all of them have failed. No matter what the price, it's not worth it.
[X] Pep talk! Tell him there's no point in moping around; wasting time like this would just be lazy. If he wants to achieve his goal, then he needs to work for it!
[X] Ask nobody for advice. You have the burden of responsibility, and it falls on you to decide rather than relegate it to someone else. Decisions, decisions.