Little info that I picked up as I played, as you earn your operation medals, you get Emmeara assult records as you complete operations that involve the units. When you complete all the operations (aka recieve an operations medal) for a unit, you recieve their assault record.
As for the Estovacean assult records, 1-12 can be achieved by shooting down the strigon team. The remaining 15 are special named units that appear in every mission and can be only found in Hard or above.
Hope this helps someone out.