What i did was use the Valkyries storm attack that requires a charge and waited til I got in a fight with a bunch of weak enemies in a travel quest. I had my party members pick them to low health so when the Valkyrie got the attack off it instant killed 3 enemies.
Warning its a charge spell so getting knocked out of it will require you to try again and you only get 3 chances on this one per mission unless you use Troop Power healing.