Pretty solid; I work with +1/+1 counters in green all the time and I would say consider Increasing Savagery . You can play it the turn after you play Marwyn for some easy ramp, and it keeps up your tempo too, and then when you can afford the Flashback it'll probably put you in endgame territory.

Definitely something for me to think about, thanks!

+1, I Love Marwyn! Have to tried out Kindred Summons ? It can double the amount of elves on the field and can pump up Marwyn for lethal, especially with Eladamri, Lord of Leaves on the board.

you should add the new Leyline of Abundance , it makes all your creatures tap for one extra mana and you can use the mana sink later when you go infinite to make all your creatures huge

Overwhelming Stampede is just plain better than Overrun , especially with Marwyn, the Nurturer s ever increasing power and Hardened Scales .

I forgot about Overwhelming Stampede existing. And I totally agree. I've thought about Hardened Scales, but most of my One drops have great other utility so I'll have to experiment.