Well if you play Sedna or Queen of Thorns you have to have a different mindset than playing with other heroes. It isn't about killing Demigods its all about map control and destroying towers. Don't get any damage items except for the ring "natures reckoning" (its a good item because you need some splash and it also gives mana) but get a lot of hp and mana items, preferably Unbreakable Boots and Nimoths Chest and Vlemish Faceguard. Minions are also a very good Idea, even the low level minions do decent damage work as decoy for the towers and the siege archers even outrange towers.

In a fight you have to attack and retreat all the time because you have superior health regeneration and protection abilities (if you play closed mode as QoT). QoT is ranged and Sedna is faster then the others so it isn't that hard.

What skills you choose depends too much on the composition of your and the enemy team, the map and the situation to really explain here.

Both Sedna and Queen of thorn are very powerful Heroes, my brother and me made it into the final of the first ESL 2vs2 tournament (final not yet played) with this combo.

QoT is all about opportunities - if you see a chance to rush in and hit with Ground Spikes, open up and do just that.  Same with Uproot etc - otherwise move back to closed and bramble shield yourself/your allies.  if you are open and become targetted, switch to closed quickly and start moving away - shield yourself and mulch shamblers to get health back.

QoT combined with Heart of Life is also great - and I always pick up one of the two high mana helms right at the start. I personally push my bramble shield/ground spikes up as quickly as possible with .  I only have Summon Shambler 1 and Mulch Shambler 1 - with Uproot next.  QoT is a tide turner - she can push from the back and with a single uproot devestate the entire reinforcement.  She is definitely not a demigod killer though - if anything she is a simple demigod distractor with bramble shield.  But thats the point - if a DG is hitting you and you are shielding yourself, you can pull them away after you have already devastated their reinforcements allowing yours to move forward. A well played QoT is annoying as hell because they will pop in, do huge damage and then walk away after popping a shield on an ally!

Also remember she is a general so having monks/priests is also useful.  Basically I havent lost with QoT in SP yet - played on easy, then normal and just now on hard with easy victories in each.  Last skirmish was 2v2 with such a comprehensive win on domination that we were IN their base when we won. The biggest annoyance to QoT Ihave found IS SEDNA and her silence - because with QoT its all about timing your spikes, your form change and yuor shield - a silence can throw the entire thing off.  But apart from that, in and out, dance and remember QoT can do massive area damage and seriously turn the tide.

I know there is a huge difference between MP and SP but I am practicing on SP while all the MP problems are being worked out. I liked Sedna before but QoT is definitely my cup of tea!

Sedna's really a party player.  Her pounce does some great damage and it interrupts so she's got a real killer there.  Her huge problem is dealing with a lot of units.  Not having any AoE abilities, she depends on her minions to take care of multiple targets.  Also, she's not very great at taking down towers as her normal attack is somewhat weak and she has no anti-structure abilities.  Play Sedna in 4v4s and stick with a buddy.  You'll get not only a lot of assists but a lot of kills too.  Sedna's high survivability comes from her ability to continuously heal herself, so if you're against a character with a good long stun, or high burst damage, you dont have much of a chance on your own: Torch Bearer, Lord Erebus, and I believe Oak are real killers for her.

QoT gets the job done so long as that job isn't killing enemy demigods.  She's got tons of ways to keep herself alive, but no way to really finish a smart player.  Once your enemy realizes he's not going to kill you, he's just gonna leave and there's really not much you can do about it.  She can creep like no other, though, so you should probably avoid other demigods unless you have a distinct level advantage, which you could very well get seeing as you can destroy structures as well as TONS of enemy creeps with ease.  Be on the field as much as you can -- waste money on mana potions, because you'll get your gold back with profit for staying out longer.  Defend yourself when getting attacked but by no means should you expect to kill a smart player without help, unless you're sporting a strong advantage.  You can also tease your opponent into thinking they can kill you, shiled, heal up and take em down once their life's low enough.  Its a game of chicken but you can win if you can read your opponent and have excellent timing.

The first artifact on the list can be a good buy for 8k gold giving you some AoE... "Orb of something or other". I tend get it if im against minion heavy generals, or if we're getting behind on citadel upgrades, but in most cases I think its better to save for the bracelet of rage or get more citadel ups. I rarely take yetis, and tend to focus on inner grace, pounce, heal and at least 1 level of silence. The "Sedna the almighty" thread on here has some good ideas in it, and following the second build posted in that thread will generally give you a really strong mid game, but you may have difficulty competing late game. 

Does anyone have any good late game ideas for sed? If everyone reaches l20 i find the battlefield dominated by UB and reg, but Heal isnt terribly useful when your ally has ~5k life so playing pure support is out. Bracelet of rage and a few minion skills seem to help a bit but you'll still get torn to shreds by the huge dps of other characters quite frequently.

Bulwark is great lategame, lots of armor and an aditional 25% reduction (that even reduces damage from skills, not that skills are that much used in late game) also consider buying the scroll that reduces the damage opponents do.

And how can you say that heal isn't that usefull just because your allies got lots of health, lots of health makes it easier to be able to heal the same person multiple times in a fight (you don't have to wait untill they are down to 10% to heal )

But this 1500 heal + debuff removal and a lil bit of damage can turn the tide of a fight. Not to mention the -cooldown aura which is very nice. Silence and interrupt speak for themselves.