I got hard achievement by going easy first, then upgrading to hard using Super Alg (Ally of Justice side mission) as a cavalry using Crystal Spear (Snow Giant side mission). Super Alg has the highest attack with a lot of speed and health. Just spam them and they will freeze any unit that lasts more than one hit. When you have a full army just spam tornado or haste to speed things up.
 "Utopia Kingdoms"){kind=icon}
 "My Little Army"){kind=icon}
 "Orbital Onslaught"){kind=icon}