Moving away from the Herald of Secret Streams/ Metallic Mimic Combo. Use Rhonas and Rhonas' Monument to pump creatures, especially River Sneak. Playing one merfolk with the monument and river sneak already in play will pump it to a 4/4, allowing Rhonas to attack as well. Nissa creates another threat and puts more creatures in play. Smooths out draws as well.
Skysovereign and the Harvester are there to block against flyers. Skysovereign offers some removal as well. We also have Kefnet's last word and Aether Meltdown in the main deck to play against Hazoret, the Scarab God, or any other major threat.
Playing against Control they will play sweepers. Negate and spell pierce should be in your sideboard for that. Your artifacts and enchantments will help you bounce back if you get swept. Rhonas is indestructible too.
Lastly, Sixth sense works very well with River Sneak and can draw you an extra card each turn, but an enchanted river sneak will inevitably attract spot removal. Blossoming Defense is in there to protect River Sneak or any other creature.
One question is whether to get rid of Metallic Mimic all together. It's just too vulnerable to spot removal, can be fatal pushed very easily. We know that we're getting Silvergill Adept. What should we take out of the deck to make room for it? Also, is there a place for botanical sanctum in this deck?
