Im sure smarter people than me will come up with some sort of convoluted KCI-style loop, but for me, Ugin's Nexus is calling, and if you get Feldon of the Third Path online, your options become insane
When this creature becomes buffed enough with other instants, sorceries, artifact equipment, or enchantments, you can eventually cast Fight to destroy the problem creature before it has a chance to attack you again
10 Archenemy Schemes: Flip the formattake on the role of the villain with powerful schemes to overwhelm your opponents
Its easily the most efficient extra-turn spell among the best blue cards MTG players can ever hope to add to their deck (its really quite expensive)
Although I already had it on my list, quite a few folks also suggested it
Sun Titan makes it into all number of white decks because theres nothing quite like it