I don't see how people can lose to Sedna. She has great survivability, yes, but she can't attack for shit.
Then you're playing very weak Sedna's. I almost if not everytime, get highest damage, kills, least amount of deaths, gold, reinforcements, buildings, and assists.
My strat for taking on Sedna, which works for me pretty much everytime is this: Using Reg, I throw mines down before Sedna grenade style, whether Sedna crosses those mines are up tp her, then I nail her with my auto attack since I usually have scope for extra range, following by throwing down mark of the betrayer.... Sedna will usually heal at this point, in which mark fo the betrayer denies Sedna some of that health, I then start throwing more mines to take out her creeps as well as her self, never relenting and constantly bombarding her with auto attack and mines. By this time Sedna will usually try to run and heal, which is time to cast another Mark of the betrayer... she is slowed down as a result and I can keep pounding her giving me the kill.
Just so you know, this build I use is heavily reliant on fast and powerful auto-attack as well as AOF. So far, it has worked
Mines can be easily avoided by just watching the outlines of them being placed, I will always attack you from the side as not to run into the obvious forward placed mines. If I do get hit by the mines, I will heal, back up while you are most likely chasing me with auto-attack + MotB, while my minions pound you. As soon as you lose a bit of heal, I will turn around and pounce you. If you try to mine again, you will miss, giving me more time to attack. At this point my regen and bishops will heal me back to full, and you will be very, very low on health. You will retreat, but by then you will get a 2nd pounce, and most likely die and/or retreat back to your base.
If you can't take her on, then go do something else like kill creeps, create a lane by destroying towers, capture flags, or get a teammate to double team her with you, etc.
Good advice. Aganist a great Sedna, you will be doing this often.