I play Sedna as my main demigod and from my experience the most powerful weapon the torchbearer has against sedna is deep freeze in combination with focus fire from your team.
Sedna's power comes from her abilities and passive health regen from healing wind, inner grace and idol priests. This makes her very hard to wear down as a TB since a fireball or rain of ice will be healed in a few seconds by her regen.
I hardly ever find myself having to back off vs TBs 1vs1. Usually I just activate a wand of speed, pounce the tb, back off regen my health and repeat that, wearing the TB's hp down. Usually Tbs just stand at the back casting their abilities on me, i regenthe health lost in seconds and they just pretty much wasted their mana.
So what you need to do is work with your team to focus fire her and her team then she will be forced to heal. Heal has a 7 sec cooldown so you need to count that in your head and just before heal comes up use deep freeze, it'll make her unable to heal for a further 10 secs, 15-17 in total and if pounce was on cooldown she won't be able to use that either. It really screws her up. Also she when she is about to heal she will stop moving and stand still to cast, you can try using frost nova/deep freeze to interrupt that, it is a 0.5 sec cast and with lag it isn't too reliable.
Now if you combine that with other stuns like foul grasps, frost nova and slows, aoe to kill her idol priests, she is going to be in alot of trouble. Just try to work with your team to pull off stun/interrupt combinations, when a team is doing that, sedna is under alot of pressure.
When it comes to dealing with oaks shield, it has a 35 sec cooldown, abuse that! Work with your team to focus fire with all your interrupts/slows, force him to use his shield on your terms. All of your cooldowns will be up before his shield is again, so then just focus fire again and he won't have it to use.
Finally shield + hol is not overpowered. All it does is provide him with around 2k health and mana every 30 secs. It just stands out alot so you notice it. If you compare it to other things like potions, idol priests, sedna's healing winds, it really is quite mediocre. If it was noticable how much healing sedna outputs, I'm sure people wouldbe whining about that.