I've played Luigi's Balloon World extensively, though some places got more play time compared to others (such as Bowser's Castle), but I have a general idea on how easy or difficult certain places are. Note that my experience may be subjective, so what may be difficult for me might in fact be easy for you. Also every place can have tricky balloons, usually because they are either tricky to reach or they tend to require quickly reaching that place to pop. Thankfully the game is designed in such a way that the seeker has more opportunities to pop the balloon than the hider do to place them.
Difficulty is graded in a five-star rating, with "*" being the easiest and "*****" being the toughest.
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Peach's Castle: *
This is the easiest level to reach most places, since the terrain is more or less open, with the trickiest high-altitude places being the top of the castle and the tower at the back of the castle (Tip: use Yoshi to reach that part more easily). The open terrain is helped by the fact that almost every place is easy to reach, along with the fact that the compass can more easily indicate where the balloons are hidden.
Bonneton: *
Unlike Peach's Castle, Bonneton starts the player from the highest point of the place, which along with the Paragoomba capture, makes it easy to reach a lot of places. Much like Peach's Castle, wherever the compass points to is very easy to identify the location and subsequently the path to go there. I daresay that Bonneton is easier than Peach's Castle because it starts the player at the highest location.
Fossil Falls: **
Despite being one of the early levels, I felt that Fossil Falls is not that easy. It's usually easy to find balloons since there aren't much deep places, yes; The player starts at a high location, yes, but there are breakable blocks that only the T-Rex and Chain Chomp can break, and if you found out the T-Rex is required, it's pretty much too late unless the challenge is restarted. If it weren't for breakable blocks, the level would be easy.
Tostarena: **
Despite being a very big location, Tostarena is not that difficult. The player starts at a high place and gains access to Glydon and electric wires nearby, which makes reaching many places easy. Because of this, I feel that Tostarena is a fun level to play due to the variety of potential places players would place their balloons. The sheer size of this location is the reason it's not the easiest.
Steamed Gardens: ***
Much like Tostarena, Steamed Gardens start players on a high location with Glydon, but I think that Steamed Gardens is more difficult because there are potentially more hidden places in which the balloon could be. It's especially troublesome if the balloon is at a high location as going to a lower place makes it difficult to catch up with the time.
Lake Lamode: ****
This is the first level that made me realise that not all levels are that easy to explore. The thing with Lake Lamode is that unlike most levels, the player starts from a location that is on the side and on a somewhat lower level, instead of the centre and a high place. This means that the farthest places are more difficult to reach, especially at the beginning of the level. It's also a water level, so traversal is going to be slow unless smart use of Cheep Cheeps are employed.
The Forgotten Isles: ***
What I find most challenging about this level is the water, or rather, the swamp. Unlike most stages where a pit is preferred as a stage hazard, the fact that a swamp is used means that there's no recovering once exposed to it, which is where most of the challenge is. Moreover, the stage starts out on a middle level, so the player has to climb higher to reach Glydon, which in my opinion would have made this place easier. I thought that this level would be tough, but I managed to get 20+ in a row so I might have overestimated the difficulty since there are like a few tricky spots, namely where Captain Toad is.
New Donk City: **
Similar to Tostarena, this is a very large location, but the player starts at a low level. Despite this, it's surprisingly not that difficult to reach most places. This is thanks to the existence of electical currents, which allows a player to reach the highest tower in a short time. Even going back to the beginning is pretty simple, so the real challenge is reaching certain buildings, especially some of the ones nearer to the entrance. Like Tostarena, I found this to be a fun level to play around.
Bubblaine: *****
Unlike Lake Lamode, Bubblaine starts from the centre, but it's also more difficult. Both levels have a lot of water, but Bubblaine also have a lot more places underwater to explore. Not to mention: there are a decent number of places above water level. The wide breadth of locations coupled with all those water makes searching for balloons an overall difficult experience.
Shiveria: ***
The player starts from the upper portion of the level, since it's the bigger area of the two. The biggest obstacle with this level is how the vision of the level is obscured, since it takes place during a snowstorm. Although the arrow points to the location, the balloon can't be seen from afar until the player gets near. The snowstorm is there for a reason: this level would have been way easier since every place is easy to reach. Therefore, the key to conquering the stage is to know the level from the inside out.
Mount Volbono: ****
There are lots of hot areas in the place, but at least it's not an instant KO. Also, there are Lava Bubbles so at the very least it's possible to travel in the lava. Some of the spots are tricky to reach so I consider it to be on the more difficult side, but there are a number of spots that are easy to reach so it's not the worst in difficulty.
Bowser's Castle: *****
I found this level to be pretty awkward to explore, because you need Pokio to reach a higher level, and the level is very big so it's not possible to reach every single location, or at least the area in the beginning. The balloon that I put in this place is still not popped, so it must be difficult for other people.
Honeylune Ridge: ***
Although this level is the final location, I think this is also not as difficult as it is. Firstly, the player starts from the centre at a lower point, but the low gravity makes it easy to reach the highest point. The fact that this level is large makes it somewhat challenging even though it's pretty open. In my opinion, the slowness of Mario's movement makes this level somewhat tedious to go through. Basically the largeness of the level is offset by how open everything is, making it not that difficult to find balloons.
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Reminder that these are my opinions. Do you feel that the difficulty reflects on my opinions with the balloons?
Thank you for reading.