Have a 5 blood mage use a dwarven hammer to build soul contracts and give them to scouts. Set the scouts to hide and let them pump out and endless supply of devils.
I would not waste blood on anything else except Ice Devils and Arch Devils, and a few select blood items such as blood stones, armour of souls, blood thorn and brazen vessel.
You will not need your national troops at all after turn 50 or so. The devils are better in all respects, can fly, and consume no supply or upkeep. Go devils!
