Sharena, Anna and Alfonse heavy arena weight

Definitely worth 5*-ing the Askr lords. Alfonse hits like a truck with a Brave Sword+, you can potentially just leave him at 4* with his Deathblow and call it good. Anna needs her legendary axe to make up her poor Atk, she like Fury + Desperation.

These are all pretty much covered on their wiki pages.

I have done the same thing. In fact, they're still 4* characters. Alfonse is the lowest.
Anna, well, she seems pretty weak to me, but add Fury 3 and I think she might actually bulk up a bit.
You should check some build ideas on this site: Gamepress.gg.

I've always been partial to polearm fighter, the bruiser. I like some axe/green fighters, but there much less available versus the blue, pike-man.

My build for Anna is purely set up for arena score. I can't remember how many points she nets, but it's a huge bump.

Anna: Noatun//Rally Atk/Spd//Blazing Light
Fury 3/Desperation 3/Threaten Def 3

Rally Atk/Spd and Blazing Light are only there to increase score, otherwise Reciprocal Aid and Luna are the better picks for skill synergy.

I have an Alphonse build as well, but my lunch break is ending... If no one posts one after a bit, I'll throw mine up there.

Hopefully Anna's helps a bit.

The easiest way to remember it for the sake of points is use their legendary weapons, use a Combo Rally(atk/spd, def/red, etc) Use an area of effect special (Rising Light/Flame/Thunder, etc)

If available, use the Drive C Skills, and have them hold a level 3 sacred Seal. Spur attack is a good one. Renewal is a good choice to give them, as it gives them as much score as Quick Riposte while being easier to get. For A skills, Fury/TA are good, if not the highest scoring. I've given my Anna TA, and few blues have any hope against her.

In terms of arena weight, both Sharena and Alfonse have the stats to run something like:

Legendary Weapon
Aether
Rally Atk/Spd (or more likely def/res, factoring in availability)
Steady Breath
Quick Riposte 3
Drive Spd 2
Quickened Pulse

For sustainable offensive tanking. Sharena and Anna also have the stats to run a similar set with Distant Counter. They can also probably run player phase oriented builds centered around the expensive "blow" skills, like Sturdy Blow / Swift Sparrow, I just can't think of any expensive offensive B Skills, besides maybe Wrath.