Block is calculated based on damage dealt, a 2 star Draugr archer hits like a truck, and block is also using your HP, so if you are not at top hp when you block a 2 * yeah it will hurt like heck.
I suggest not tanking the archers or range stuff, just los kite them and take the melee out, then murder the range.

To tank and block, you need high HP AND a good tower.