I've probably played that mission too many times to be able to properly judge the difficulty but I can give a few pointers on how I do it.



I pretty much only use the nuke and the fire ball
A ballista tower goes down with two fireballs
Every time the nuke is up you fly towards the next island with a big central tower and use it to wipe out a bunch at once.
Ignore the zep and just fly ahead.
Don't fly in straight angles and use speedburst to change direction often.

and finally something that saves you a lot of time and well makes it too easy is that the zep is really strong against flying enemies and can take out the spawning towers without any problem so you can mostly just ignore those and focus on lightning/ballista towers


Hope this helps those with problems on this mission.