I can't really comment on where to level it at the moment... but when it gets Xeno Slasher, I hear good success stories of Eira in that water dungeon.... Brasilis I think it was?
But yeah, the attacking skills of Eira aren't really all that great.
Since OD2 has people on Ymir, I have it cast skills there (Eraser cutter 1shots all, or leaves orc archer crippled for 2~ melee attacks), and Xeno Slasher kills mobs.
The cast time is abit long on them, so I opt for Lv2 to make it more bareable. But even then Xeno Slasher is still abit long.
Are ant eggs free on your server? Cause those are afkable without relying on a server to fill a map.
Sadly, I guess you can't really afk much in your server... so you won't be able to make use of overed boost's intimacy farming for unlimited afking.
So yeah, I guess you need somewhere to grind until you at least get Xeno Slasher.
(Tip: You can use XenoSlasher -> Chaotic Blessing -> XenoSlasher if you require 2 casts to kill a mob)
(Tip2: Mob with Overed Boost, 500 Flee should make you survive pretty well if things are on the Eira. And XenoSlasher has uninterruptable cast time.)
PS: I made eira, and kept it for a long time because I liked the look of it.
While leveling, I was so jealous of my friends' Sera and Dieter.