Yes, each island will generally have harder stacks than the last, so it does become difficult as you go. Some general advice: you are rewarded for winning fights without losses with extra leadership. I think it is +200 for 10 fights, +500 for 30 (i.e. an extra 300) and +1000 for 50 fights. I counted 20 fights on Debir so if you can do them without loss, it will make Scarlet Wind that much easier.
On Scarlet Wind and Bolo there are stacks that are much harder than the rest. These are the map guardians and the bounties. I found I had to leave 4-5 of these until a bit later. So there will be some stacks on Bolo that are easier than the hardest ones on Scarlet Wind, and some on Rusty Anchor that are harder than the map guardians on Bolo etc.
Another cool trick is to get the first stone (stone of Hope?) without fighting on Bolo. You have to pick up something from the underground lair there and it is being guarded by a boss stack, but if you are sneaky you can say you want to fight and then dodge the stack when it runs toward you. Then it is a piece of cake to grab the thing (I forget what it is) and take it back to Debir. That's worth another 200 leadership, I believe.
