The most important thing about treasure hunter, is to just keep practicing, and realize that one wrong move has about a 50% chance to break the entire board. Also, not all treasure hunter boards are created equal. Some board layouts generated are likely to cause a stray block to wind up stuck alone in the middle of a fish/monokub, and some boards let you clear out an entire monokub in 2 selections. These are gonna be five very generic tips, but I'll throw them out there anyway since they're what helped me the most.
#1 - Even if you flub your board and are 100% positive you will not get your S-rank on a given attempt; keep doing your best on the board anyway to get some practice.
#2 - Take a break. If you've been going for hours and start doing worse, take some time to go clear out some characters in talent plan with a podcast on or something. I dropped it after my first 6 hours of attempts, and only came back to it after beating the hidden boss in the dungeon. I managed to clear it in 3 attempts with a fresh mind.
#3 - Pound the color (or number of circles in the blocks center) rotation order into your head.
White (1 Circle) - Pink (2 Circles) - Yellowish Orangish (3 Circles) - Blue (4 Circles) - White (1 Circle)...
#4 - Once you have a handle on how to set up patterns on the board, work on adjusting the blocks on the interior of a monokub/fish to be cleared out as much as possible, while trying to leave as many blocks connected to an objects edges as possible. The more blocks connected to the edge of a block you want to clear, the more options you have when it comes to finding a way to remove it.
#5 - Your First move is what's going to bite you the hardest, even if you don't realize it. Don't remove a set of blocks that will leave a block stuck alone for this first move. I don't have enough fingers and toes to count the amount of times I saw a ton of blocks on my first move all stuck together, and then realized i either isolated a block on an object, thus immediately locking it; or fudged with the other blocks colors so much that other matches became harder.
Good luck doods!
Edit - Oh dear, first post in this forum, and I overlook someone linking to an advice thread already. Mistook it for this forums style of in-topic quoting. Oh well. No harm leaving this post here anyway I suppose.