How to build +HP/-ATK L!Tiki?

Aether and call it a day.

Just keep her base kit and give her Reposition/Rally skill, the QR seal and then Aether. If you can't afford Aether, use Bonfire. I'd say Ignis, but Bonfire allows you to proc a special with Bold Fighter while Ignis won't proc unless you run Quickened Pulse/Ostia's Pulse. If the QR seal is already being used, you could try Panic Ploy to take advantage of the HP boon or something like Close Defense/Distant Defense(HP/Res or Atk/Def work too, for example).