I found that loading up Blitz Firefight with Kinsano and the "Inferno" card in your deck is the quickest way to kill enemies, try to use at as often as you can, since the enemies are usually quite grouped up when they spawn, it's best to hit them just after they leave their base. You could either let yourself lose once you reach Heroic/Legendary so you can you kill enemies much quicker with the ability, or keep playing and just replay sessions of Firefight until you unlock it. From my experience, it's much faster than trying to obtain it in Skirmish.