For the merchant problem: if he won't sell you stuff it's likely because you have too many items for your (inventory) side of his sell screen. Use up some consumables and projectiles, then try again.
The jumpy platforms need to be more lenient on timing, I failed 5 times before learning to click half a block early. Also, beating the avoidance sections with keyboard control is only possible if you use the dash button (Shift) or auto-dash.
After hours upon hours of staring at those darned colors and fish segments I finally had the clarity of mind to glance at a statue and go "wait what is he doing". (I swear that wasn't there before!)
...and I thought I could beat this game without ever seeing the word "furthur"! Jokes aside, I think Amber's powers caused one of the swings in the park to disappear.
This is truly high difficulty. Some jumps are almost pixel perfect, with a precision of ~0.1 seconds on the start and end. You can have >90% of your footprint on another block and have it not count, as long as it's the one on the right. Restarting on the spot takes almost one second, restarting from a level fail takes 2.5 and uses a different key. And it takes me >50 tries to even 3-star the first few levels!
Those who say this game is too easy have never tried to solve the 8x8 2-4 puzzles without trial and error. If you thought having multiple solutions was bad, try having to account for that in your solution procedure, especially when the 3s aren't independently solvable from 2s and 4s.
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Hi, which gate?