This is a 1v1 situation verse a Sedna. You can counter her with anything but another Sedna.
The idea is to win the match by being superior in a 1v1 scenario.
This situation takes place on Crucible, the game is Conquest.
Your warscore is 5 and Sedna's warscore is 4.
Sedna has currently spent 6700 gold, this includes the initial 1000.
Early game you got a kill for 600 gold hooray!, so you have 7400 gold at your disposal for this situation. This includes the 1000 starter gold.
Our Sedna:
Currently lvl 8
Build:
Heal 2, 500 mana, heals 900 hp.
Healing Wind 2, 24 HP per second.
Inner Grace 2, 15 HP per second, 10% movment.
Pounce 2, 525 mana, 600 damage.
Favor Item: Blood of the Fallen, 800 hp, 5 HP per second
Items Bought:
High Priests (oh exploitable)
Banded Armor
Unbreakable Boots
Nimoth's Chest
Vlemish Faceguard
No citadel upgrades
Basic Info:
HP:4700 MP:3400
Armor: 1300, 33% weapon damage mitigated.
Attack Speed: 1.4
Avg Weapon Damage: 225 (Unmitigated)
Health Per Second: 60
Her High Priests heal for 1500 every 10 seconds.
Your Info!:
You're lvl 9. you've got 7350 gold.
No deaths, one kill.
Skill isn't a factor.
Remember, this is a Conquest match. And you cannot counter with Sedna!
-Foreshadowed 
PS, Sedna has 6700 total gold including the intial 1000 because she sold monks for 700.
Lemme know if you think our Sedna's build is lacking or if something needs to be edited!