Urgh, I'm getting frustrated in general at the game right now. It seems like Defense is the most important stat, and most of my units don't have enough of it; most enemies carry silver weapons and kill almost every unit I have in 2 hits.
Birthright does not have nearly enough tanky units. My two best tanks are Silas (Great Knight) and Rinkah (Oni Chieftain), and even they lose huge chunks of health to most attacks at this point.
It seems like the name of the game is killing everything before it kills you, because trying to weather attacks from more than 1 enemy at a time is asking to have your most vulnerable unit targeted and killed.
RAAGE
Obviously I'm not that far into Birthright so I can't give you level-specific advice, but here's some general advice:
-Abuse partners as much as possible when playing defensively, having a partner *always* negates the team attack from enemy units, ontop of occasionally negating the primary attacker as you already know.
-Be certain to effectively use characters that have passive support abilities. You've undoubtedly noticed ones like Sakura with "allies within 2 spaces take -2 damage", that helps a lot, especially since if you have several characters with abilities like that, they can overlap and stack onto one character, making them especially tanky.
-Abuse chokepoints as much as possible, just like Leonidas says, it doesn't matter how many Persians there are if they can only attack you one at a time. As an addendum, having the person blocking the choke be *bad* at killing people, since if they're good at killing, then on the opponent's turn your blocker will kill a guy, letting the person behind him come in to attack, and he'll kill that guy, and the guy behind him will get an opportunity to attack, and they'll get as many chances as they need to wear down your blocker.
-Never split up your army if you can help it. Having your whole army as one big death ball lets you steamroll fights with sheer numbers and attack power surprisingly often. Remember, you never need to defend if your opponents never get a turn.
-Great Knights have superior defense, but Sila's other promotion option, the Paladin, learns an activated ability that halves most forms of ranged damage, so remember you can class change and learn other skills if you need to, then switch back. Alternatively, you had the option in character creation to spec out your Corrin to be very defensive, and can learn to be a knight IIRC, just having a beefy main character smooths over a lot of fights because Fire Emblem AI has a tendency to prioritize attacking your main character over anyone else.
-If you haven't tried out Conquest yet, you can try that. Nohr as you might have gathered has most of the 'traditional' Fire Emblem classes, like knights, which will let you assume a more defensive playstyle more easily.
I'm looking forward to the special challenges that Birthright might give me, since tanking in Conquest is very easy with how awesomely defensive some characters are, but it seems there's simply no tank class in Hoshido's ranks, so I'll have to adjust my playstyle to fit that.
The only instance where the game doesn't play by it's own rules is with that one guy in Conquest Ch10, but I'm OK forgiving that because it's to prevent breaking the story, but honestly I'd have preferred they resorted to a legitimate method of preventing you from killing him, like surrounding him completely with overleveled promoted units, giving him the skill that lets him do what his cheat does (which he has in a later chapter, so wtf there?), or just completely omitting him from the map so you can't actually interact with him, just don't break the rules of the in-game universe please.
According to Fire Emblem Wikia, Takumi does have Point Blank on Chapter 10. He doesn't on Chapter 13 for some reason, but he's not cheating on 10. :v
The reason it doesn't break the story is because basically any enemy that's plot relevant but gets killed instead goes "nuh-uh, that didn't count!" and runs away. This is perhaps most notable with Ike in FE10, where if you kill him in 3-13 (praise the archer) his plot armor magically lets him survive wounds that would kill him if you were playing as him.
Heh, maybe I just completely missed it. I'll have to eat my shoe for being so inattentive.