I'm trying to get level 20/20 with a lot of characters to transfer to RD... but, the only chapters that I've found good to Boss Abuse are 11 (against the Knight blocking the Arrive place), 16 (the boss), 21 (Ena) and 23 (Petrine)... there are any others? It can't be chapter 26, because Bertram uses both Physical and Magic so I can't abuse him...
While you're reading this, millions of people are dying now...THEN DON'T READ THIS!
If you don't want to take the chance, swallow your pride and flee the fight. I realize this sounds disheartening, but consider: the Black Knight is full of crap anyway.
The treacherous coward claims he's big stuff 'cause he trounced Greil and chased Ike around - but the lamer was invulnerable to their attacks the entire time. I can't help but notice that he didn't have the guts to square off with Caineghis! He's all like "I don't need no help" to the soldiers in Nados... yeah, that's real brave, coming from the guy who murdered poor untransformed Lotz from behind. And even if you do your level best to shove a sword through his head now, he's so much of a cop-out that HP -> 0 will not be allowed to kill him. It would teach him a lesson in humility, except that as we all know, he'll just pretend that he lost on purpose. I bet he'll even believe it.
Granted, denying the BK what he wants means missing out on Nasir, who is cooler than Ena. But I don't think either one of them is going to be a good candidate for transfer bonuses, so... is he hours cooler?
Ena is almost certainly your best option. She has the stats to not be OHKOed even by a critical by most characters, and her weapon never runs out so you're getting 2 phases worth of damage if your damage isn't too high.
Asides from that, Renning is fine, you should survive okay at first and eventually his weapons will break (you can steal one anyway).