Fishdom 3: Special Edition

On the right side of the screen during levels, you'll see a bobber dropping lower as time goes on. The gold cutoff is about a third of the way down and the silver cutoff is about two thirds of the way down. To unlock this achievement, you need to finish a cumulative total of 100 levels within the gold time limit, which you'll also get a gold time bonus for.

You should be shooting for the gold time on every level, and most levels aren't too hard to get the gold time. Some will require you to use explosives to progress fast enough, such as levels with lots of chained or frozen tiles.

You're going to have to play upwards of 500 levels to reach level 48, which is your ultimate goal. It's highly unlikely you'll make it that far without achieving gold time on 100 levels, but if you do, there's a workaround for this achievement. Go into settings and switch on Relaxed mode. This removes the time limit from levels, so every level you complete will count towards this achievement. You could do this right from the start, but I don't recommend it. Doing so removes the time bonus you get from completing levels, significantly reducing the amount of money you get. Since you need money to level up, and leveling up is the rate limiting step, you're just slowing yourself down by doing this from the start. If you don't have this achievement by the time you reach level 48, then you should switch on Relaxed mode and earn the last of the money you need toward buying ten sea turtles, and all levels you complete will count towards this achievement.