Combos Utilizing Maro

Opportunity gives you the extra cards to make Maro big and the library prevents you from discarding them(multiple Opportunities work best)

Play Endless Swarm once you have Skullclamp and Spellbook on the board. On your next turn, 'clamp as many tokens as you can to draw a ridiculous amount of cards, thereby getting you even more tokens. Maro is just icing on the cake.

Get the Maro out, then geddon, lots of cards in your hand

With Library of Leng you can have many cards in your hand, and Barrin's Codex gives you many cards at once to make Maro a huge creature.

use properity to get all the cards you want

first, play spellbook, so you have no limit to cards in your hand, play maro, and then upheaval, making maro totally unstoppable

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allows you to draw 1-5 cards, strengthening the maro, depending on how many colors are in the deck.

first play Maro, then play Howling mine and Library of Leng to get a big creature