Ossian Grr aka "Josh"
Thanks for all the clarifications so far, one questions that would help.
If one monster is blocked then the entire attack is block.
So if non of my creatures have breakthrough and I send 3 13/13 monster and the opposing player blocks with a 2/2 creature, it blocks the 37 damage left over?
But remember you can attack multiple times per turn.
In this situation, it's generally better to attack with your 13/13s one at a time, so your opponent can only block one of them.