Deafeting the Black Kight

Well, what I did was Have Ike Attack the Black Knight from a ranged position, so Mist can stand behind him, out of the Black Knight's Range. On your turn you just wait, but on his turn, he will attack Ike, so then on your next turn, heal Ike and wait again.

Remember that attacking him from a distance is useless because Ike's Aether skill won't activate, and you wont have time to kill him. Just attack him, one-on-one, and hope for an Aether to activate. After it does, you shouldn't need another Aether to activate (although it will hasten tings up quite a bit) unless Ike's Strength is seriously lacking (unlikely). Then you should be able to attack him until he dies. It's better to defeat the Black Knight because Ena is drastically underlevelled (level 10), whilst Nasir is level 18. And it's the "true" ending of the chapter, as we find out in Radiant Dawn.

Note - The Black Knight never attacks Mist; only Ike. However, the reinforcements attack Mist and the Bishops will also heal the Black Knight, so it's better to eliminate him as soon as possible, preferably by the end of the third turn (after which the reinforcements arrive).

Get ike to equip the knight ring, give him aether (or vantage or cancel and adept and miracle if you think you're lucky) have ike attack the bk from a distance the move him up to the bk get mist to use a long distance staff on ike if you trained her which you should) if bk activates luna then move back one space and get him healed then attack him from a distance twice then get him healed, then move him up and by then if you're lucky and aether has activated he'll be dead (after turn 3 you might wanna kill the bloody bishops that show up but I've never had them show up soo go figure that part...)

Hm.. I guess I got lucky since I defeated the dark knight after the reinforcements could attack mist. Make sure Ike has the occult scroll used on him. I killed him in about 3 or 4 hits, and in 2 turns, the aether skill will activate again. I kept mist behind a pillar, but the knight never moved towards her. Just get mist to heal him maybe level her up a little with the bonus exp, get her a mend staff just in case. Btw. I wouldnt suggest leveling up Mist to a Valkrie since her damage sucks and in about 2 hits, the reinforcements could kill her. And do not attack the black knight from a distance, I'm pretty sure he also has ranged attack too

I just beat him today, with a lvl20 ike (of course) but also with a lvl 20 mist(Valk) I wasnt lucky enough to get aether to trigger till around turn 5, however by having a lvl 20 Mist, she can kill the bishops easy, easiest way to get her up without wasting bonus exp, is to buy ALOT ( I got around 10) ward staffs at the start of chapter 17, I may not be right but I think thats the only time you can buy them, and by spamming ward when there is no one hurt to help gain exp for fast lvls, also good for Ryhs. A lvl 20 mist can also take more than a few hits from the Halb guy and easily kill the Bishops, mine only had 15 str, and sword skill of C, but had a Forged sword (Steel sword 4 MT -3 WT 9 Crit 10 Hit so all you really need is wep lvl D and she comes with that, all together thats about 25K-35K and thats including about 10 ward staffs). But others get lucky with Aether triggers, but I didnt and was happily prepared. Its a bit expensive, but if your Aethers dront trigger it really helps, and the Black knight will not attack Mist, as such the Halb and Bishops will not attack ike. Spoil alert if you defeate him you gain Nasir(lvl 18 white dragon), lose and gain Ena (lvl 10 red dragon), it may not be in this game, but I know Red dragons are good against Melee but have terrible Resistance, while White is vice-versa.
Enjoy.
Hope this helps someone.

Ike should have the abilities miracle and aether.And don't worry about Mist.The Black Knight never attacks her if Ike is in his range.

Alright one of the best strategy's here is too buff up mist to the point where she can handle herself. Do this before this chapter comes by giving her alot of bonus XP when she gets to that point she becomes uber and can take out everyone that jumps into the room to help the black knight.
As for Ike just constantly attack the black knight in close range as much as possible while healing with mist when necessary.