Lets say she gets some rediculous healing and defense numbers:
1500 self heal every 5 seconds
500 heal from minions every 5 seconds
200 hp/sec regen
= 600hp/sec regen total
Say her defense is high enough that it cuts your damage in half, it would give her effectively 1200hp/sec. Thats pretty stout.
With a game that has gone on for 2 hours, you should basically have your pick of artifacts.
Bracelet of rage + ashkandor + deathbringer are the necessities, other than that you need an additional 15% haste to be at the speed cap, from there you should work on +damage and life leech.
At that point your base dps would be ~1400dps, enough to kill her in 40 seconds assuming she has ~8000hp and doesnt use potions. With potions she would still be very difficult to kill.
Pop bracelet of rage for an additional 300 base damage (which at the speed cap is 600dps), bringing you up to 2000dps. This will have her dead in 10 seconds (the same duration as the ability... funny that).
Pop beastial wrath at the same time (+65% damage) and your doing 3300dps. Enough to kill her in under 5 seconds, at that point healing potions wont be able to keep her alive.
Sure sednas tough, but if you cannot kill her after a 2 hour game as UB its because you built your character wrong, or bought the wrong items. You really dont even need venom spit to be able to put her away.