As you duel other players and win, you accumulate credits according to the rewards listed in the appropriate forum. However, when you win a duel, depending on the tier ranking of your opponent's deck in relation to your own, your rewards will be affected by the following multipliers:
Two tiers below your own: 0.25x reward multiplier
One tier below your own: 0.5x reward multiplier
The same tier as your own: 1x reward multiplier
One tier above your own: 1.5x reward multiplier
Two tiers above your own: 2x reward multiplier
As for the tiers, taken from elsewhere:
Tier 1:
X-Sabers
Gladiator Beasts
Blackwings
Machines (Scrap, SalvoDAD)
Fairies (Perfect Herald, Counter Fairies)
Tier 2:
Gadgets (Anti-meta, Machina Gadgets, etc.)
Plants (Debris-Hime, Quickdraw, Gigavise, Naturia)
Monarchs
Lightsworn (Pure LS, Twilight)
Infernity
Gemini City
Dragons (Disaster/Hopeless, Dragunity)
Tier 3:
Everything else. Voice all complaints loudly.