Elvish Spirit Guide

Exile this creature from your hand: Add {G}.

Llanowar Elves

Elvish Mystic

Timberwatch Elf

{T}: Target creature gets +X/+X until end of turn, where X is the number of Elves on the battlefield.

Might of the Masses

Target creature gets +1/+1 until end of turn for each creature you control.

Heedless One

Trample
Heedless One's power and toughness are each equal to the number of Elves on the battlefield.

Forest

Fyndhorn Elves

Instill Energy

Enchant creature
Enchanted creature can attack as though it had haste.
{0}: Untap enchanted creature. Activate only during your turn and only once each turn.

Imaryll, Elfhame Elite

Whenever Imaryll attacks, it gets +X/+X until end of turn, where X is the number of other Elves you control.
You may have Imaryll assign its combat damage as though it weren't blocked.

Concordant Crossroads

Wirewood Pride

Target creature gets +X/+X until end of turn, where X is the number of Elves on the battlefield.

Jagged-Scar Archers

Jagged-Scar Archers's power and toughness are each equal to the number of Elves you control.
{T}: This creature deals damage equal to its power to target creature with flying.