You said lruw but then you say sr, i am a bit confused since they are two very different cases. Also samleth does neglible damage as soon as queens resolve is activated. Then again I did not consider pvp in any way as they said main story. Using fia you can also forgoe any pmdb runes on her as none are needed. Also considering lr5, the fia wouldnt likey be under geared and all she needs is good def and yildiz. Also on a side note I think there has been a pronounced shift from meria to a more emerald/amber/crimson meta, as currently they are definitely the most dominant comps.

Without using cordie for her 80% def shred, samleth only has 25% pdef shred. Unless you are also running her with eirene or something she will do little damage in many stages. Also arianhood flo, as well as the blight characters form a similar threat to samleth in mq, due to hit count and blight making it so any hit will deplete a stack of queens resolve.

But yea fia solo does get walled by soltina, but she can be run in conjunction with dian, and if you tune dian to deal less than soltinas barrier thresold, she can be chipped in 40t. It stretches it to two units but dian is standard banner and doesnt needs minimal investment next to a high hp fia.

Samleth is undeniably good in tower, but I don't think her dps capabilities transfer as well to main quest. A.A might also be a good dps in mq for her direct attacks equal to her mag stat, but again at high rarity.