So, you've gotten pretty good playing NoMess at level three or higher. You're working up the nerve to play well at a really high level of difficulty. I suggest instead, that you start at level ONE (applicant), for a really exciting long game.
Level one is too easy, you say? Well the challenge here is to collect bombs at the first few levels, so that you will have lots of bombs to make the higher levels easier to play. If you enter level four (1500 points) with seventeen bombs on the screen, you're in better shape than if you just started a level four game from scratch.
Hoarding bombs presents some interesting challenges, because they can get seriously in the way. In general, you want to keep the bombs that fall on the outside squares of the board, and slowly use the bombs that fall in the middle. At levels one and two (and even three!), you should hope to have at least one bomb on the edge for every hundred points you've scored. In some games you will hoard bombs considerably faster than that, but chances are you will have to detonate some because they will block your open lines. (You CAN have too many bombs.)
You can make it more likely that bombs will fall on the outside squares by keeping those squares open. Whenever you have a reasonable choice, move a piece off the edge to form a three, four or five in a row, but don't overdo this. Clearing the outside edge is anti-positional in NoMess, because you will have to clog the center (instead) and block more open lines. It's considerations like this that make the long game exciting, even at level one.
With practice you will get a feeling for how much you can let the board fill before reaching for your bombs. I tend to be pretty free about using the bombs in the middle, and try never to use a bomb on the edge until level four (well, almost never).
To get a feeling about how well you are doing, count the pieces on the board from time to time, ignoring smileys and bombs. Remember that you start the game with seven pieces on the board. If you count when your score is, say, 1020, and find you have ten pieces plus nine bombs, you are much better off than you would be, STARTING a game at level three. If you enter level four (1500) with three bombs and twenty pieces on the board, you are unlikely to do as well as you would if you started a game at that level.
You can also try hoarding smileys, but I think it's more valuable to keep using them as needed to keep the board open.
How long is the long game? You can aspire to reach level seven. My best game lasted 2,260 moves.