Terastodon, Woodfall Primus, and Primeval Titan are probably the best cards for the "big green creature" role, but Simic Sky Swallower and Broodmate Dragon can serve the role as well, although they require greater deck manipulation to be able to hard cast should you draw either of them before finding your Natural Order
With how quickly you can amass a large and wide battlefield, you can start getting in for damage before your opponents have a chance to set up
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Don't get me wrong, I love Kaima -- Auras are one of my favorite archetypes and Goad is one of my favorite mechanics, and I even own a paper Kardur, Doomscourge Goad deck -- but Goad decks are inherently a low/mid power strategy: very strong against Combat-focused decks, weak against Combo decks, and impotent against Sacrifice decks