Fire Emblem: Radiant Dawn

First, swap out Haar for Jill, as she has the better speed cap. I'd also reccomend bringing another lance user so you can use the Wishblade, like Nephenee, Tanith, or even Fiona if you know how to use her. Mages are terrible for endgame, so only bring Soren if he has an Ike support.

I believe Triangle attacks are overrated. They are entirely situational. I suggest giving Boyd a forged Silver Axe instead of Arbalest, and give Oscar the Wishblade or a Forged Silver Lance.

The following is what the Official Radiant Dawn player's guide says:

All of the mandatory ones(obviously), and:
Caineghis
Giffca(w/ Laguz Gem)
Naesala
Nailah
Skrimir(w/ Laguz Gem)
Tibarn
The better of Aran or Nephenee
The better of Laura or Rhys
The better of Jill, Marcia, or Haar
The better of Leonardo or Shinon
Rafiel

That's not absolute, just a recommended.

Yeah if you've got the royals you dont need to worry, but for the other characters you could try nephenne(wishblade) rolf or shinon(i prefer rolf, but if you wanna be cheap go with shinon, double bow)reyson(none),mia(vague katti or alondite), haar, (jill if you want, even in their own ways) soren(rreally good mage), and for a healer micaiah wil do just fine.

The best team I've ever used was:

Ike: Ragnell
Maciah: RexAura
Mist: Florete
Mia: Alondite ( w/ Wrath + Resolve)
Sothe: Baselard
Soren: RexCalibur
Sanaki: RexFlame
Nasir: Breath
Naliah: Great Fang
Caineghis: Great Fang
Tibarn: Great Talon
Naesala: Great Talon
Leanne: None (Lv 40 :D )

From my experience all you really need to beat ashera is, obviously, the characters you have to have. which means you'd have to have ike at a high level in order to beat the black knight. And then its as simple as Caineghis, tibarn, giffca, nailah, skrimir, mordecai, and any other laguz you want. you get so many superpowered laguz its easier to just use them than try to use beorc. of course it is all about preference, so using my and other's advice, do whatever you see fit.

Here was my party:

The required ones and
Caineghis
Tibarn
Nailah
Volug-Laguz Gem
Haar-Silver Polaxe
Boyd-Urvan
Mia-Alondite
Soren-Rexcaliber
Pelleas-Balberith
Aran-Wishblade
Shinon-Silencer
and Rafiel

You have 17 units listed... you can deploy 17 units in the battle. What are you asking us to do?

I think that Naesala's kinda repetative, you don't really need him.
Stefan too, as VK has higher might than Alondite. You can just keep the Alondite on your trueblade and use it as a ranged attack.
I have to agree, swap Haar out for Jill, and add in Nephenee.
The last character can be any ridiculously strong character, including Boyd, Nolan, Laura, Rhys, Ilanya, or any other character that you really like.
The triangle attack isn't really necessary though, I'd just stick with Rolf/Shinon, or maybe add in Pelleas if you are on your second playthrough

I say bench Sothe because Volke is much much better imo (higher str cap for starters) and i agree with Pelleas; triangle attacks are overrated. From my experience, as long as the Beorcs have a ranged SS weapon (so no VK) and are a decent character, you're good to go.

My final team was:
Ike - Ragnell
Soren - Rexcalibur
Micaiah - Rexaura
Edward - Alondite + VK
Rolf - Double Bow
Boyd - Urvan + Forged Hand Axe (Mt 15)
Ellncia - Amiti (but only healed for the whole 5 chapters)
Tibarn - Great Beak
Volke - Baselard + Peshkatz
Kurthnaga - Breath
Reyson
Nephenee - Wishblade
Lehran

For some reason I hate using the max number of people so I just stacked blessed weapons on people (i brought Stefan for VK, Jill for HA, and Sothe for Peshkatz).

I have:
Ike - Ragnell
Mist - Alondite
Soren - Rexcalibur
Mia - Vague Katti
Boyd - Urvan
Nephenee - Wishblade
Micaiah - Rexaura
Sothe - Baselard
Sanaki - Rexfame
Pelleas - Balberith
Rolf - Double Bow
Tibarn
Naesala
Caineghis
Nasir (After he joins)
Kurthnaga
Reyson

That team is fine. You don't need to change anything.

It really depends on who you raised. The team you selected should be capable. Just remember to give Nihil skills to your best attackers, or else you'll be taking a lot of damage against Ashera.

Here's my personal favorite crew, balancing strength with some interesting character dialogue. There are improvements that can be made, but it's adequate. If dialogue/events do not concern you, feel free to axe Soren, Pelleas, and Naesala. Nasir is also usable if anyone in your crew isn't working out (or if you're not using one of the required characters), but do not bother with Gareth; he is entirely too fragile against magical opponents.

Ike - Ragnell
Micaiah - Rexaura
Sothe - Baselard
Sanaki - Cymbeline (Check Sanaki's Strength; RexFlame is most likely too heavy for her)
Kurthnaga - Breath (Bonus Experience and Satori Sign gets a strong fighter)
Ena - Breath (give her some Bonus Experience and the Imbue Skill; she is an outstanding wall)
Naesala - Great Beak
Tibarn - Great Beak
Caineghis - Great Fang
Nailah - Great Fang
Soren - Rexcalibur
Pelleas - Balberith
Elincia - Amiti
Marcia - Wishblade
Jill - Urvan
Mist - Alondite (in all honesty this took some RNG abuse for her Strength; I just have an obsession for both of Altina's swords being with Greil's kids)
Leanne

Yes, I picked the "underdog" of the Herons, Leanne is the most usable without special attention. Reyson is awesome with a Laguz Gem. Rafiel is awesome with Boots/Celerity. But I like to give those items to other characters (that and I'm a sucker for the Leanne/Naesala ending). Take your pick.

I went without Giffca because I prefer giving the Laguz Gems to the Dragons. That and his lack of boss dialogue are the only conceivable reasons to bench him; he is awesome.

Yeah, basically what Hypershell_SBL said, except I'd personally change the team around a bit.

If you want a bit of a challenge, (Assuming you're playing on normal/hard, seeing as the Dragon's are pretty tough if you're unprepared.) I recommend bringing in a third Laguz into the Party just because of the insane stats that they get transformed. Also, this is just a Guideline, you can work around it as freely as you want. But these are considered some of the best characters in the game. (Well, excluding Vika/Astrid because they just take way too long to train.)

I also recommend sacking Oscar for a different unit (If he turned out bad.) Boyd (If Nolan turned out better than Boyd, but really they're both amazing units. And really, unless you're gonna play smart, Triangle Attack isn't gonna be used too terribly much in E-1 to E-5) Stefan for Mia (More time to train = usually better stats at the same level.) Haar for Jill (But really, that's up to you, seeing as they're both great.) Naesala for a Beorc that turned out amazing.

My team at the end ended up like this. (excluding the required. Characters.)

Nephenee - Wishblade
Nolan - Urvan
Rolf - Double Bow
Soren - Rexcalibur
Jill - Forged Silver Axe
Vika - Beak (SS Rank)
Astrid - Forged Silver Bow
Edward - Vague Katti
Mia - Alondite
Elincia - Amiti/Psychic Staff
And then for the Heron, I added Rafiel w/ Celerity. (Seeing as how I killed off Leanne twice, and Reyson once.)

Nephenee had Nihil #1, Ike #2, Nolan #3, Edward #4, and Vika had Parity for Ashera.

I went without Royals because I thought I would be better off without them. (4-E-3 decided to prove me wrong.)

I just use the Laguz royalty, Giffca Ike with ragnell Micaiah and sanaki because they must be there And then the strongest of the remaining characters