As far as vampires go, Klaus is unbeatable without some kind of magic enhancement. But witches? Witches are just OP as fuck. Even an average one will beat most vampires. Originals are still strong enough to beat most witches, but Davina in this video is far more powerful than a witch should be because of some unfinished ritual that gives her the power of five witches.
Only a witch which posses power of multiple witches can restrain Klaus, not actually kill him. Klaus has both vampire and werewolf genes, so he is like two in one. They once needed power of 100 witches to even hold down Klaus during his transition to a werewolf vampire hybrid, but with Davina, she had the power of three other Harvest witches. And Davina could never even kill him, because killing Klaus would mean everyone he has turned die. After that power went away, she was no match for Klaus. And even if you manage to subdue him, you can't permanently kill him.
