Successfully complete Campaign mode on the Pilot or higher difficulty unlock the PAK-FA. Then, go 
to Free Mission mode, and sortie the PAK-FA with Trinity (EW1) on any mission that allows multirole
aircraft (the "Siege" mission is recommended). Fire the Trinity at an enemy (for example, the two 
groups of enemies at the end of the "Siege" mission) to get the "Calamity" achievement.