Hello, yesterday I was playing on a custom map, and there was creeper inside of unbreakable ground. Is there a way to destroy it? That map has scores of people beating it, too.
Might help if you tell us which map. :)
If there's a blue X over it there is most likely a gateway that will remove the ground, make sure there's a path from the gateway to the ground without anti-creeper if this is the case (and don't nullify the gateway before the dig packet arrives). There might also be fields that take the creeper out of the ground to somewhere else, either time or digging another block can start this. The last option is to get creeper near it and hope the splash damage of launchers kills it. If you tell us which map it is, we can give you more detailled help.