Actually the 301st card is the card you get for the achievement, so you need to get every last single card in the game to get that achievement. Pretty brutal even for those of us who try to keep up. Locking a card behind Memoria Misera, an extreme trial people have no real reason to farm and is quickly becoming dead content was a pretty dumb design choice. Card should have been added to the Imperial Triad Pack in 5.3.