Fire Emblem

[Aozaki-desu]

Lysithea should very well survive it, so its not a problem. At worst you can use a shield and see if that helps or if it forces a more favorable target.
Your Dimitri clearly needs more defence. Give him a Aegis Shield and Duscur batallion = +16 defence. He is the tankiest out of 3 lords and in my playthrough he took basically no damage after timeskip (except that one time that Hilda killed him but thats just Hilda being Hilda).
The map is very anti-dimitri but its because its mage-heavy and he is bad against magic, which is fine because Ingrid is very good, or short of Ingrid your Byleth should be able to Retribution the shit out of any boltings / rush the artillery units. If getting in range of ballista's proves too difficult BL has Sacred Shield batallion.

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You can also whip out 5 HP talent on your Lysithea

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Oh and suffice to say you can also focus on getting a unit into a throne room to force her to switch in melee mode

[GDB]

Again, normal hits are fine. Super easy to survive. Random ass crits for over 100 are not. Even if I beef up Dimitri like that (not sure I have the Duscur battalion leveled or not, don't care enough to turn the game back on to check), the second hit attacking someone else critting is still guaranteed to kill them. 5 HP isn't doing anything against that level of crit.

I'd try just plopping someone in the throne room, but those meteor mages basically mean whoever goes there has to be able to tank an absurd number of magical attacks.

[Aozaki-desu]

Pure water
Ward (dont stack, use one of)
Impregnable Fortress (you'll need to rescue after but thats trivial)
I never had Edelgard critting my mage (maybe once, i dont remember) but thats just one divine pulse
or again, impregnable fortress
or sacred shield because its a physical ranged attack

[SirGauoftheSquareTable]

Dimitri being bad at Resistance is literally the only reason I ever had him as a Paladin for awhile. I just wish Cavalry classes didn't have shitty Speed growths while Fliers are OP. I just don't understand that decision.

[Bloble]

Impenetrable Wall plus Warp will make anyone invincible. Throw them in the throne room. If your Flayn has any levels she'll have Rescue to yank them outta there too.

Alternatively just put only high luck units in Edelgard's range, give them stat-boosting equipped items, or use Manuela's personal to give everyone near her 10 Crit Avoid to reduce the crit chance. You don't need to clear the entire map, just get into the throne room and choke some points while your heavy hitters chunk down Edelgard.

[SirGauoftheSquareTable]

For reference, us this an NG+ playthrough or a from-scratch one?

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Also, do you have Thyrsus? That should help Lysithea's survivability.

[GDB]

From scratch. And since my first thought when reading Thyrsus was "Who's that?", I'm going to say I don't have it.

[SirGauoftheSquareTable]

Bummer. That's the Relic you get upon completing Lorenz's Paralogue that is meant for those with the Crest of Gloucester. Lorenz can use it, but it's best suited for Lysithea since it has a Magic Range +2 effect and can halve any incoming damage (triggers Guard).

[GDB]

So, managed to mostly mitigate the stupid crit RNG by putting whatever weak unit is being targeted next to Manuella (lately it's been Flayn). Lowers the crit rate by 10% basically. Cleared out all but the right side, and moved on to Edelgard's room, and... suddenly 2-3 reinforcements per side pour out of the stairs. Normally that'd be whatever, except they all appear to have Bolting. So literally cannot be anywhere in the throne room except right next to Edelgard, basically.

No idea how to deal with that and it kind of tapped out any of my recuperated patience with the game that I got over the past week.

[SirGauoftheSquareTable]

Stair reinforcements are annoying in any difficulty. I get you. Probably need to block off the stairs ASAP, though I don't know how helpful that advice is. Also, Retribution would likely do you a huge solid with the Bolting mages.

[GDB]

Can't block them, I tried. They just file into the next open space and there's not enough units to block all of them.

Retribution might help, if they didn't always target the weakest units.

[SirGauoftheSquareTable]

Do you have any Silence users? Also, nothing to do about that targeting the weakest. As a consolation, only one strong attack breaks Hegemon Edelgard's barrier, so it's pretty easy to defeat her if you get there quickly enough and dogpile. Also, Manuela definitely has Ward, which likely will help against the Bolting mages.

[Bloble]

Ward, Res-boosting shields and battalions, switching to high Res classes, swapping in durability-boosting skills, using that "take 1 damage" ability on them... there are plenty of options to shore up squishy units.

And someone with high Charm (like Dimitri) can easily neuter a bunch of mages at once.

Really though, same-turn moving reinforcements with siege tomes is cancer and I hate that 3H thinks it's okay to do this just because divine pulse exists.

[SirGauoftheSquareTable]

This is why I love the Aegis skill enough to suffer through Paladin reclassing...

Seriously 3H, why do Cavalry classes have to suck so hard? I literally only feel comfortable making Lorenz one for any real length of time.

[GDB]

Okay, looking it up, it seems like only the first wave of Bolting Reinforcements can't be stopped. Once those are dealt with, parking someone on the stairs allegedly stops more from appearing. It'll be a hassle, but this seems doable now.

[Aozaki-desu]

This is why I love the Aegis skill enough to suffer through Paladin reclassing...

Seriously 3H, why do Cavalry classes have to suck so hard? I literally only feel comfortable making Lorenz one for any real length of time.

Half of them are good though, its just that other half is so offensively bad

[SirGauoftheSquareTable]

I guess Holy Knight and Bow Knight are good, but are any others? Also, that goddamn Speed growth debuff...

[Aozaki-desu]

Paladin is fine
it has 8 move, is available early and its requirements arent too bad, and it has lancefaire. Terrain Resistance kinda sucks but oh well.
Speed penalty is just something you trade for Canto and ability to field a good batallion
Just like fliers trade good batallions for superior stats and movement type
its not perfect balancing but its logical and reasonable

Bow Knight is fine for range, canto and batallion although for archers i still very much prefer snipers. Or i guess if you are willing to sacrifice offense for canto to use stride or some other support batallion, but there are healers for that.
Valkyrie is the best mage class in the game despite the speed penalty. Doubling with tomes is something a very few mages can do anyways.

Holy Knight sucks, but for a pure healers like Mercedes or Linhardt i suppose there arent really better options. Mercie can also go for it at her own pace after getting Valkyrie which is a boon. Sucks to be Linhardt though, can even get its requirements to 100% before he hits 30, and Sothis forbid you also want fiendish blow on him.
Dark Knight just sucks, i guess it exists for Hubert.
Great Knight just sucks major dick with no redeeming qualities. Paladin is straight better.

[GDB]

I like Holy Knight for Flayn, since she doesn't have a long range heal outside of Fortify. So having Canto to let her run in, spot heal, and run out is nice.

[Aozaki-desu]

Flayn is a pure healer so yes she is good as a holy knight
however, i prefer Dark Flier Flayn because fling rescue is better than canto rescue