Not sure what difficult level author is playing on, but archers are complete trash past normal and any dragon units (including Nah) are incredibly powerful in the midgame (only outpaced in lategame/DLC maps because their super low speed cap prevents them from double attacking anything).
In normal, who cares? You can get through with mostly unpromoted units.
But yeah, on topic, FE has never done permanent death well. It feels hollow, forced and very very artificial, like they heard once on the phone what difficulty meant but never quite grasped the concept in full. Protip; replaying the first 20 minutes of a mission because 4 guys spawned within walking distance of your backline units and got kills on the very first turn before I could even react is not "challenging," it's "utter bullshit, go the fuck back to game design 101 before making any more shitty games."
FE could learn a hell of a lot from XCom, too; the earlier FE games with fog of war were absolute bullshit, because it was like the spawned units but you just couldn't see the entire map and thus a single unit could get picked off from 9 spaces away from gargoyles flying in from waaaay out of vision range. In xcom your choice of cover actually matters, and the game gives you tools to deal with unknown enemy locations and lots of fog. FE didn't, and suffered greatly because of it. At least they figured that out and didn't put any fog in awakening - at least one person in that company still has a working brain. He needs to lead an armed insurrection to overthrow all the dipshits that think permadeath from spawned units/walls opening up at random is good design, cos that shit is still holding FE back from being a truly great series. It's right on the cusp; a little improvement could go a long way. But they keep bringing back all the stupid bullshit from the earlier titles in service to brand, completely ignorant of what makes a "good" TBS game.
You want to blindside and permakill units? Okay, let me recruit them from somewhere. You want to throw fog of war everywhere? Okay, give me cover to hide behind (and 1 castle per map that the boss is afk on doesn't count). It's not that FE is a "hard" game, because it really isn't; most of the strategy is dealing with the outdated rock paper scissors faffery and making sure your dudes skirt around the edge of enemy attack range so they don't get buttfucked. The game just becomes complete shit in a waffle pan when they decide "hah, fuck these players, I'm just gonna kill that healer because I'm feeling vindictive, and there's nothing they can do about it except soft reset and play that first 20 minutes over again."
Fuck you, Fire Emblem: Awakening. When you actually get good, give me another call; until then I'll be playing far superior TBS games.