Triple Triad questions

How exactly is everyone progressing so quickly? People already have crazy cards like Bahamut. I keep hearing something about taking advantage of Plus rule. I tried that by playing the easiest Plus opponent Jonas. I'm also not sure I understand the Plus rule fully.

Even if I did understand my cards aren't that great and buying new ones are expensive. I have been unable to defeat any NPC except Triple Triad Master. The expensiveness is not a problem. What is is figuring out how people have big cards so quickly. Are these people just getting very lucky in GATEs and getting big MGP? Right now I have all of the starting cards, plus I bought a Gold pack which gave me a 2 star card named Gerolt, plus Spriggan I won from Triple Triad Master. If Gold packs are 2 star, then what is silver? I thought bronze is 1 star.

Basically I'm asking what others did to progress. Technically I could just play Triple Triad Master over and over for 5 MGP and earning 10 MGP back, but that would take ages.

They drop from certain encounters. A FC mate of mine got a garuda card this morning from doing garuda ex.