Whenever you cast a creature spell, create a 1/1 red Mercenary creature token with ": Target creature you control gets +1/+0 until end of turn. Activate only as a sorcery."
: Choose a color. Add one mana of that color for each different power among creatures you control.
Flood the board with creatures of varied power to tap Selvala for explosive, often game-ending mana, then dump your hand and snowball Mercenary tokens off every creature cast. You ramp hard, go wide, and convert that mana into huge X-spells, big payoffs, or a deck-out/combo finish. Turn to turn you prioritize diversifying creature power values so Selvala's tap ability scales fast.
Massive repeatable mana once you have a few differently-statted creatures
Mercenary tokens reward creature-heavy curves and quietly add bodies and power
Refills your hand off 1/1 Mercenary tokens, fixing the deck's card-advantage problem.
Add untap enablers (Instill Energy, Magosi-style effects, or Seedborn Muse) and mana doublers to break Selvala into infinite or near-infinite mana, then add a clean payoff like Walking Ballista or Finale of Devastation. Prioritize creatures with distinct power values and counter/anthem effects to maximize her tap, and include strong protection (Heroic Intervention, Teferi's Protection) plus card draw (Skullclamp, Beast Whisperer) to offset her lack of innate advantage. Tighten the curve with efficient mana dorks so you can deploy creatures and tap her the same turn.
Legendary Creature — Elf Scout
: Target creature you control gets +1/+0 until end of turn. Activate only as a sorcery."