Can't say I encountered any difficulties, I just advanced as usual.
Though it's quite possible that I have played often enough that I already know the best strategy for most situations and avoid all difficulties. It might still be a good idea to try out strategies before looking at the spoilers.
Strategy for the small islands:
Spoiler
There's enough ore to create a foothold where you can build relays.
Strategy for the large island:
Spoiler
Five Berthas? Against some low-to-medium emitters? You need no friggin' strategy, auto-target is sufficient to keep the creeper down. Just send sprayers and guppies to supply them and you're done - I sent two cannons and two mortars as well, just for overkill, but I doubt it's necessary, the Berthas will make short work of digitalis so you don't really need anything that shoots.
Of course, given the large island is the endgame, maybe no difficulty was intended.