TB has a tendency to focus on burst damage, not prolonged damage, so I'd say he can have the hardest time killing her.  That said, he does have two rather good tools in his belt, both in his ICE form: Deep Freeze, and Ice Nova.

The Deep Freeze can count a heal while it is being cast, and/or prevent one from being cast.  Most of the time, deep freezing her is the same as a silence for the duration listed (5,6,7 seconds) in addition to possible damage.  Hit it when she is looking like a heal.

Ice Nova, on the other hand, will stop her priests from healing for the duration. Fire Nova will also work, but insta-killing them, but doesn't have the nice stun on sedna at the same time. This can actually deny her a lot of her healing for a decent period of time.

Also, remember that you don't have to kill her to win. Unless it's Slaughter Mode... then you are perhaps, F'ed.

deep freeze deep freeze deep freeze!...wait you were fire? Uh....She'll run out of mana eventually!!!

I rarely have it used on me but Warlord Punisher can really piss me off when I am playing Sedna especially against hit and run demigods like the TB. The damage is slightly annoying but the mana loss basically takes a heal away in the early levels. If you stack it with Staff of Renewal it can make Sedna's at least have to go back to the crystal more often and if they aren't watching their mana carefully then you can kill her.

as fire tb: buy monster upgrade, and kill her monsters.     

 the object of game isnt smash into another demigod and fight until one of you drop.  the object of the game is to destroy the enemy citadel

She's vulnerable to stun ganking.  An Erebus-UB team can make short work of her with a combination of charm-bite and foul grasp-spit.  She may be close to impossible to take down 1v1, but she can go down quickly in 2v1 and 2v2 scenarios when she's the main target.  She doesn't do as much damage as ohter DGs, so so if you burst her down to 2/3 or lower health with stuns, she will usually have to flee because she's not going to be able to take anyone out.  Other DGs have ways of escaping that Sedna does not (QoT and Oak shields, Reg has mines and run, TB has freeze, Rook has boulder, Erebus has swarm, UB is just plain fast).  A HP heavy Sedna will be lower in speed and vice versa.

I have had some problems killing Sedna with probebly all DGs except for ooze UB. Once I get to lvl 2-3 of ooze the added DPS and the reduction of attackspeed for Sedna makes her not being able to stay long in a fight. I can also position the UB so that the ooze kills her minions.

Another way to beat Sedna is by going Towerrock but it aint that fun and if I miss boulderroll and slam she can escpape most of the time.

My general suggestion on how to kill her is to have items that reduces her attackspeed and stop her regen. AoEs to take out her minions are helpfull aswell.

One tough combo is playing vs 2 Sednas on Cruchible, that makes for some challange  

He did say late-game, so if it was Fire TB, there is no problem sustaining damage as you get pretty much infinite mana after level 15.