You get monster cards from Digimon depending on the level of it
Level 16-27 Digimon drop level 2 Cards
Level 30 Digimon drop level 3 Cards
Level 40 monsters drop level 4 Cards
Level 50-60 drop level 5 Cards
Level 70 drop level 6 Cards
Except Megadramon at Ruined Historic as they drop boss cards only they do that.