First, without cost reducers we don't net any mana
Treasure Vault No colorless card has really made a huge impact on a standard yet, but Treasure Vault is a monster in Commander and Modern
Any time you have a creature enter the battlefield with Arwen in play, that creature gets +1/+1 counters equal to Arwens toughness
That permanent gains Haste, When this permanent leaves the battlefield, it deals damage equal to its mana value to each creature, and At the beginning of your end step, sacrifice this permanent. Terminal Velocity is a bit of a mish-mash of different effects
While Counterspell is the most well-known, Id recommend Arcane Denial as the must-include counter
Listings with PSA notation should be viewed cautiously