Feat
Stoneskin
One tick of skin that a weapon does not care for
The Monk drives blue mana outward instead of forward. The skin hardens for an instant and is soft again before the instant ends.
DR equal to 10 × the Monk's level, for one tick only.
It is taken as a reaction, which is the whole point: the Monk sees the blow coming, spends the Ki, and takes it on stone. At Level 5 that is DR 50 for a tick; at Level 10, DR 100. Nothing a smith can make competes with it, and it is gone immediately.
Spend 1 Ki point and a spell token. The token comes out of the same two spell actions that Ki Strike wants, so a Monk cannot spend a round both untouchable and unstoppable.
This is what a Monk has instead of armor—and unlike armor, it does not slow him down, does not spoil his dodge, and does not stop him from meditating at night.