Make her fast with accuracy, so she can land the debuffs before your damage dealers step in.
Also, probably worth keeping your evolved Warmaiden for Barbarian Faction Wars, and use a freshly farmed dupe from campaign for fusion - at least, a dupe you've not used to skil up your developed Warmaiden. :)
Why is fusing Rhazin the reason not to bring Warmaiden to 60? Rhazin is a very good champ, but his decrease def is only single target, so he can't replace Warmaiden. Rhazin has a totally different profile. He can work in arena as aura lead for go 2nd teams and has heavy single target damage + debuffs. Warmaiden has AoE decrease def.
There are good reasons to leave Warmaiden at 50, namely shard pulls of Dhukk, Tayrel, Stag Knight, Deacon, Uugo, Daithi. But Rhazin is no replacement for her.