Is my -Atk +Res Lene usable?

Sure, she's a dancer. She'd be usable with a 0 attack stat.

A dancer is still a dancer. If you want her to be a combat-support mix, then I would recommend keeping Safeguard+ and refining it with either +Spd or +Atk, while giving her Deathblow 3 and SwordBreaker 3. Then give her some mixed buffing skills like Hone Atk 3 C Slot and Hone Spd 3 S Slot.

Personally, I would give her Pavise as a special as well to enhance her survivability. Or if you want, turn her into a buff bot with this set...

Keep Firestorm Dance for +Atk and Spd buffs, but then give her Fortify Def 3 and Fortify Res 3 in C or S slots. That way, you can give any one character that she dances for +3 Atk/Spd and +4 Def/Res

Hope this helps.

A -Atk dancer might sometime even save you from death since it's the most useless stat on them since they just need to dance and survive. On dancers, spd followed by def and res are your most important stats so that's actually a pretty good Lene. (If she gets demoted, which she probably will, you can still change IV's, so don't fodder her off!)
~FV