Making a map 256x256 doesn't make it difficult. This map, is quite simple when you know how to nullify the 2000 emitter on the first island.
Spoiler
A guppy + Freeze+Convert+2xBertha to destroy digitalis
After that you start....clearing up the creeper for the rest of the 1,5 hour.
If you want to make challenging maps make them much smaller, look at J's Tiny/Micro/Nanoland series. All hard maps, all very small. Instead of piling emitters just add enough creeper at the beginning so that the player have hard time setting up the base/expanding. Usually emitters at 20-40 are enough.
I made a guide about using the mapmaker on steam, you might want to read that, although you seem to know how to use the editor quite well^^
Anyway looking forward to your next attempt.
Concur with pawel345.
No real challenge here, just patience and a willingness to spend the time working though it.
Please look through some of the really highly rated games that have been submitted for ideas of what appeals to the players.
Thank you for taking the time to make this.
H