The total number of pieces is twenty one. Break this into three sets. Seven Strong, seven middle, seven weak. Within the 7 strong, 2 are spies and 5 are generals. If you manage to split the spies, the game will be simplified into a one on one manuevering race towards flag capture or the finish line.
The essence of the game is basically memory or position recall then maneuvering. If you can recall where a certain strong piece originated as well as the pieces surrounding it, you can almost detect the remaining, This can be simplified by actually challenging or testing each. By the principle of deduction, you should have pinpointed the opponent's flag even during the middle if not towards the end game stage.
Basic rule is to maintain a one is to one ratio of pieces. Do not assume anything. As long a piece is unchallenged, it could be the flag. The only way to find out is to test it. The more pieces you test and recall, the nearer you will be to finding the flag. If you are down on quantity, you may be in trouble even if you are up in quality. Not unless you are taking a gamble by assuming that a particular piece is the opponent's flag and ignoring the remaining.
If the flag is at a corner you only need 2 pieces to trap. If it is floating somewhere near the corner or last row, you have to manuever it to a corner. For every piece that's protecting it, you will need another piece to even up the spacing because you can only move a single square every turn,The quicker you pinpoint the opponent's flag, the harder it will be for him to win. If the opponent's flag is surrounded by all his strong pieces, initiate separate attempts to cross on opposite sides of the board. This will force him to spread his cover to deter your advances.If he ignores your attempts, drive your flag across instead of capturing his.