It is not possible to overcome a 1,000 intensity emitter. Mortars remove only 4 layers of creeper per shot. You would thus need 250 mortar blasts per second to remove the creeper emitted in one second.
It doesn't work exactly like that because a shot (mortar or blaster) also damages a number of cells around the cell that was hit directly. Attached test map proves that 5 blasters can (barely) cap a 100 intensity per second emitter (while they only do max 1 damage per shot). In a confined space the creeper can't spread, which makes blasters even more efficient. The creeper from the second emitter (1000 intensity per second) is successfully hold back by just 7 blasters (for a while, as the blasters get damaged).
Of course this doesn't mean that the rejected map can be finished (I don't know what happened during the 7:28 minutes it was played), so I agree that a score should be submitted.