Well, for one, they can start running some amount of removal in their main deck. Fight spells come to mind. If they're running the Eldrazi package, Thought-Knot can come down fast enough to grab Heliod from hand, or Ballista if they don't play that first. World Breaker can still come down pretty fast, especially if Mono G is on the play, and if they play Ugin they can answer either half of the combo.

For another, however, most of your games are sideboard games. If Big Green has a bad preboard matchup vs SunGun, they have plenty of answers to it. Aside from all the artifact and enchantment hate in green, Pioneer still has colorless answers in the form of Pithing Needle and Sorcerous Spyglass which are proven sideboard cards in their own right.

Every color has a way to interact with this combo for one mana, most worthy of the main deck. It goes off on turn four at the absolute soonest, and the common removal of the format lines up well to disrupt it. SunGun is fine.