Your BLM powers depend on your ability to hide/be ignored.
If you are ignored, you can cast 3 spells in 10 seconds - That's like...7500 dmg, plus a swiftcast 4th if you got it which should kill something without vit acc.
However...that's 10 seconds of having some person not moving and eating it. Also they must not press their CD. Also they must not be healed. That's a very small opportunity window.

Now if you are not ignored...just run away until they get distracted and come back for a whammy swiftcast thunder 1. Then Thunder 1 their pets/turrets/friends. Then TC whatever runs away to instakill something with 10% or less HP. That way you can feel like you're still worth it. That's what I do. I hand out a 6 second nap here. A 6 second nap there. Fish for easy kills and feel better about this really really bad PVP job.
Once my anima is completed I can switch back to a real job that feels rewarding and less broken.