Copy of: +1/+1 counter / Hydra cultivation (Modern)

Mirrorweave is a one sided wipe if you target one of the many (0/0) creatures in the deck, because every creature without +1/+1 counter dies. Furthermore it`s a great finisher if used on an attacking Kalonian Hydra.

There is rarly a creature on your side that won´t start with a counter, thanks to Plaxcaster Frogling, The Great Henge,
Llanowar Reborn or receive one in the following turns.
That enables a better mana ramp (Incubation Druid, Gyre Sage), card draw (Fathom Mage?,Inspiring Call) furthermore protects every creature from Mirrorweave and enabels Protection (Plaxcaster Frogling, Inspiring Call)

Llanowar Reborn`s Graft is mainly to accelerate Incubation Druids ramp (turn 3, 6 mana).

Hadanas's Climb can double your attack damage in the round it was played, if you target a creature that has already 3 counter (easy), after that you can tap for it's transformation ability.

Muddle the Mixture allows you to search for anything you need in the deck, Ramp search for Incubation Druid or Gyre Sage, Finisher? Any of the Hydras, Graveyard hate take the Scavenging Ooze, Artifact or Entchantments the Steelbane Hydra is your friend.

Slots to change:
if you are not on budget another two primordial Hydra, Kalonian Hydra(even more thanks to Mirrorweave) or Hydroid Krasis.

Alternative for the Gyre Sage you can use a Vigean Graftmage for an infinte mana combo with the Incubation Druid.

Gargos, Vicious Watcher alternative would be Rosheen Meanderer easier on the field 4 Mana for Spells with X cost, that includes the Stonecoil Serpent better but excludes the Kalonian Hydra.

Optional:
Planewide Celebration is a nice to have and is never dead in the hand, life gain against burn decks, an army of creatures that can be boostet with Oran rief, the vastwood or "just" 4x proliferate to improve every single creature.

Oran-Rief, the Vastwood to generate more Counters.