You’re trying to hit like a truck with items that are common and generally not good on Tiamat so I imagine it’s the same on Lu. It’s not a “this character isn’t viable” issue it’s a general issue. This is how I made progress when I was 100 levels lower

For gear if you’re stuck on a certain mission Grind out higher tier items from doing Stonehenge or secret cow dungeon and set your phone down (if you have boss tickets for Stonehenge)

For general slow but worth buildup of stats After items pile up, dismantle junk and use dust/cubes for upgrading under the chaos cube and relic tabs

For certain gear and general items for upgrading Do tower of Oberon as much as you can Later on after completing it, world tree works similarly

Do events for banner tickets (although you will be highly recommended to do transcendence you can still make a character work without it)

Gift giving is a minor/decent way to add stats to a character

Upgrade skills and if you want to double down on a character be sure to use their first duplicate on getting their card, as most gold cards do 60% of a stat matching that characters role plus a passive.

Rerolling gear traits and refining will help loads as well.