Disrupt it when it tries to heal. It's not hard to guess when a Sedna will heal, once they drop below half you should already be reaching for that stun skill.
This is one of the biggest fails I've ever read.
1) The cast time on heal is faster than several of the stuns.
2) Even if it weren't, the 250 ms forced latency dramatically reduces your window of response.
3) Even if you could respond fast enough, there isn't a casting animation, so you can't tell when the heal is happening in the first place.
Again, this isn't a general imbalance. In other game modes, various other factors come into play to balance things out. It's only in the capture the flag mode where it becomes a problem.