The cost reduction applies only to generic mana in the total cost of blue spells you cast
Waters In short, you can either go price hunting on an MTG card trading site like TCGplayer, get some MM3, MH2, MH3 packs, or wait for future reprints
Rhythm of the Wild makes your creature spells uncounterable, but giving your creatures riot pretty much guarantees a +1/+1 counter if youve already got your other haste enablers online
Putrid Goblin comes back to the battlefield with no counters on it, and you can sacrifice it again and repeat this process indefinitely
Ready for the previous multiplayer powerhouse that would have topped this list in the old days that's a bit diminished in Commander hence it hitting my Honorable Mention space
Mill two cards, then destroy the chosen permanent if its mana value is less than or equal to the number of cards in your graveyard