Dualcaster Mage is one of the most played red creatures in EDH
Seven mana is a big ask, but in combination with Sun Titan, Karmic Guide, Burnished Hart, Solemn Simulacrum and Fetchlands, you can recur your whole board constantly
Similar variants can use Walking Ballista substitutes, like TriskelionTriskelion, and other counter increasers like Lae'zel, Vlaakith's ChampionLae'zel, Vlaakith's Champion
Spider Spawning is significantly more difficult to deal with than cards like Bloodline Keeper (which can get smoked by a Brimstone Volley, bounced by a Silent Departure, hunted down with Prey Upon, etc)
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If the card is a permanent, it's considered in play, unless it's a local enchantment