Heres your problem. When your opponent gets too farmed up, they tend to get hard to kill. Sedna, is a god with move speed and heal spam, and CAN be unkillable, if you allow her to be. If you had played the early game correctly perhaps, they wouldnt have leveled as much, or farrmed as much to be so unmovable and quick. I know exactly what you're talking about, and I'm pretty sure I know how to beat it as well.