"Hello. I have been sent by my Master to offer you a proposition. If your organization will work to kill the Einzbern association, we will not kill you at this point in time. Do you accept?"
Einzbern, one of the three Great Families. A family of master alchemists, who in this city have a worrying amount of clout.
The golden-armored hero looks to you expectantly, awaiting your answer.
Your castle is destroyed. Shirou is still slightly injured, and Cu's pool of accessible prana has shrunk from the leylines of Fuyuki down to his own prana.
In other words, your situation is hardly an advantageous one, particularly against the hero who had just destroyed your castle moments before.
You're still certain you have enough prana to use Gae Bolg several times, or to activate a series of runes, without any issue, but is that worth the risk?
 "Okay, we'll accept your deal. We'll go after and kill the Einzberns." (truth)
 "Okay, we'll accept your deal. We'll go after and kill the Einzberns." (lie)
 Mentally tell Cu to fight. He just used an A+ NP, he has to be below top form. You can win. (Specify: Gae Bolg, runes, straight combat)
 Try to distract the Servant with small talk while you try to get a clear head of what's going on, maybe find out his Master.