For the life of me, I cannot work out this achievement.
How do you engineer a 500 block chain in Tidalis...?
Unlike most of the achievements in the game, this one requires taking advantage of clever tricks. The board can only be 12x12 spaces at its largest, so typically the largest chain would be 144 blocks, but there are a few gameplay features you can use to clear blocks from the board before their chain completes. This can free up space that you can then use to add more blocks to the chain. I suggest looking at the various special blocks, gameplay modes, and items for things you can use to achieve this. You'll want to create your own custom game or a puzzle in the level editor for attempting this achievement (though if you make your own puzzle, make sure you play it outside the level editor. Stats in the level editor test mode are NOT tracked).
If that's not enough of a hint, there is a thread on the official Tidalis forums that discusses this very achievement and includes some diverse strategies, but I'd prefer not to make it too obvious.