I'm going to have a supplementary article about "easy" upgrades, that is, Beta Reward cards which only cost 3 Cards to upgrade to Level 2, instead of 5! These will come out soon, after I finish this series of Top 5 Upgrades for Newcomers!
But basically, any card that has that 'smooth' looking card surface, like Gelatinous Ooze, only costs 3 Cards instead of 5 :3 You can snag some quick Level 2s and 3s with those.