How to 'best' use Eureka?

I unlocked Eureka last night - I spoke to her over and over over the course of months to build affinity until she told me she wants Junk - which I had a vendor so i just spammed junk at her - took almost 4 inventories full then killed her in 1 frame

My current setup is:

Me - Wraith Swordsage using Muramasa

Fox Maid - Riding her she has Masamune from dwarf

Older Younger Sister? - the green hair chick, hosting her using Holy Lance

Defender - guy just follows me healing me

Cleared Nefia 97 currently

I did one dungeon and Eureka cannot keep up she is too slow, she stays half a screen behind. Im thinking of hosting her and getting Older Younger girl to follow me, infact it seems vastly more inefficient to have additional npcs who cannot only do range attacks as they stuck in single file hallways

Also im confused about 'levels' of companions. Eureka is 35 but has no trait points and decent hp but terrible PV even with gear on so im assuming she is also level 1 - is her 35 her 'base' level? meaning she eventually will get much stronger - my other npcs have been eating food from a level 90 innkeeper with high stats so they actually have more hp than her currently

Eureka stat panel says she is good with Scythes but her combat skill lists bonus to mace/sword/spear and not Scythes - I would give her the Lance but the other girl has been using it for a while - I dont know what weapon to give her

I am mainly playing melee and will push it as far as i can