Lene help

That's pretty goods since Dancers want more bulk than offense, with +res she will be able to live longer, not by much though since it's just +3 but she will survive better if you focus on her def and res.

Refine her sword with +res, if you want a budget build then give her Fury and a +spd/def seal.

Dancers don’t stand on the front lines, attack is irrelevant. You could get res refine for extra bulk and run stat boosting skills such as fury or simple stat ups for a more bulky kit. Otherwise, a spd refine could also work to prevent doubles.
Wings of mercy and some kind of support C skill, drives, hones, tactics and fortifies could work.
Then you basically have a decent dancer.
More expensive skills such as close def, close guard or a defensive stance skill could also be used if you are willing to sacrifice units for them to build Lene
(Though it’s not really that worth it unless you really like the character)

Lene can get really good mixed bulk if you want.

With Safeguard active, a res refine, and Fury for budget stats, yours can get 33def/38res. And then just toss a generic +def seal on her to even it out a bit. Or, since you have -atk, you could just dump it completely and use the Fortress Def seal for an even 38/38. She shouldn’t have to kill things anyway.

If you want her to attack powerfully at all, you’d want a special like Bonfire or Iceberg. Pick whichever is the highest stat, or whichever you have more copies of lying around if it’s even.

I personally like Miracle as a defensive special option, but it may be hard to charge. You could experiment with the shorter cooldown defensive specials, too, but those specialize a bit much if you want a general tank.

I highly recommend Fury for the A slot. It’s cheap and gives lots of varied stats that don’t rely on specific enemy attack ranges. For a more expensive option, you could use Steady or Warding Breath for the special charge and a bit more specialized stats.

Wings of Mercy is always a good choice on dancers, but her native Firestorm Dance is neat for buffs and doesn't overlap with the Earth Dance seal if you want even more buffs.

You could use Guard to deny enemy special charge when she’s hit. Vantage can be useful, too. Renewal could make her more tanky over time.

The B slot is pretty open for experimentation. Use whatever you like the sound of best.

For the C slot, put whatever your team needs. If you don’t know what her teammates will be or don’t plan to use her on a set team, I recommend something like Drive Speed. Usually good and a common pull.