seriously.
You are still in act 1 and complain about mechanics.
Yet you haven't even seen half of the game.
rupture tendon doesn't scale with crits thus it's high base damage falls off at some point.
you will see its effectiveness fall rapidly from lvl 17+
it still does good damage, especially cause it's piercing damage - which is why it has high cooldown. But sawtooth blade will become even stronger.
Alexander already at lvl 16 will have ~ 4 or 6k life not sure anymore and almost the same in both armors.
he keeps healing und ressurecting and blessing stuff. A ruptured tendon + chicken combo alone won't even cut it remotely.
the witch isn't even a good example. She is a relatively weak "boss". her adds usually make the fight hard