"And what can constrain the power of a being who created the whole multiverse? Anything in the multiverse would by definition be lesser than you. And yet, you managed to get defeated by them? Tell me, can you make a lie so implausible that even you can't believe it?"
"I was bored. Essentially, I let it happen. Perhaps out of some remaining fondness for this place, in order to keep myself from destroying it. Now would you like to see, or not? I assume that you have some way to look into memories. I will allow you to look into mine, as far back as you can stand."
"Like you can't fake those. As I've already noted, you're more powerful than I, so I have no doubt you could fake memories better than I could detect them, if you were so inclined. Without calling on the Lord of Nightmares--technically possible but not an ideal option--there is no way to verify your memories."
"Then let me make this very remedial for you. I am what I am. I don't have to answer to you, or to any of your cronies. So deal with it. I'm not going to tell you anything but the truth just for the sake of your precious ego. If you can't deal with that, then press the issue and see if I don't smite your ass."
"Fair. Incidentally, outright lies are rather crude and crass. I would have expected a being of your stature, whatever it is, to care about such things." Xellos shifts back to his normal cheery demeanor (eyes closed and all), opens the door, and goes onto the deck.
Selina gulps and tries to get Angel back in control via bracelet so she doesn't set the ship on fire...
[4+5v10] Eh, let's go with the most dramatic option. After a moment's struggle, you manage to get Angel back. Halfway through a Flare Arrow.
Angel gets burnt. Lina looks horrified, and goes over to Angel, spilling apologies. She promptly heals the damage she did.
"Nasty trick of that Nuriel person to play. Next time you turn into her, let me know, okay?"