When Magic: the Gathering went back to the plane Innistrad in the set Eldritch Moon, it brought with it a terrifying new mechanic called meld
Or make it very costly for someone to kill it, either in combat or with a kill spell
Better still, it's an X cost creature, costing two green mana plus X, where whatever number is assigned to X will generate that many mutagen tokens
Space-ic Lands & Galaxy Foils
For creature matchups, we have a motley crew of blue cards, with So Tiny being our Sword to Plowshares, Warrant // Warden often permanently answering a creature since we can put it on top of our opponent's deck and then mill it away, Run Away Together offering some fun tricks where we can bounce our opponent's best thing and one of our enters-the-battlefield mill creatures, and Flood of Tears being our (very bad) Wrath of God