Exactly how large is this battlefield? The Moonchasers should have line of sight and even if they are out of primary range, he definitely is still within range of ballistic fire.
Actually, unless it also takes Viserys and company 10+ turns to reach him, he should definitely be in the range of direct fire.
Hmm... you are right, the Moonchaser can fire on it, though it is still going to take the people on the guns a bit of time to come up with a firing solution, they are after all only human and they did not get a heads up from Dany. Call it two turns. Twelve seconds to gape at the newly manifested godling before you start shooting it sounds reasonable to me.