IMIW Auto-Block
In AW, fighting IMIW boss with Corvus.  Prepared to one shot him by punishing his heavy attack, have him down to 5%, 3 Glaive charges left, SP2 ready, hit the special and he auto-blocks it for almost no damage.  Bait heavy, auto-block, repeat this last cycle 6 times in a row with auto-block.
Question is how did he auto-block and unblockable attack?