If you have the time, could you please share a few words on the strategies we would apply to win?

I really tried to find it in Store, but not succesful.

hmm, thats strange. This link works for me. (comes the first one).
https://play.google.com/store/search?q=sixfighters

I tried it. I only beat it on the easy level a couple of times. Didn't try anything more difficult.

Nice little game. I thought it was good combination of simplicity but allowing you to calculate in a pretty simple and straightforward way. On the other hand, if you aren't paying attention, it's easy to get caught out which makes it challenging in a fun way.

My comment would be that the rules seem unclear to me. I had to play around a bit to find out that the elimination could work sideways (presumably backwards also). The rule also seems to imply that whatever the orientation the man stepping into position for the elimination has to come from an adjacent file or row, but in game play it appears he can also step up to his friend on the same file to achieve the elimination. Also, it doesn't matter whether the space is behind the enemy man or behind your own pair of men, but this wasn't clear in the rules.

Also, I realize it doesn't really fit with the rules, but after I saw the variations above, I expected the following to work,
0XXX ---> -XX- (that is in a setup with 3 guys in a row and an enemy at the head, moving the back guy away doesn't work to eliminate the enemy at the head.)

Maybe it's me (I checked, 12 wins against hard). I was a bit surprised that my mobile is apparently communicating with you. I just learnt something. There was no prompt before installing (usually I get prompted that an app needs access to phone directory, ID, bank account data, youtube & browser history etc, which usually means that I don't install the app. I was very happy that your app didn't ask for these special powers.). So this seems like standard procedure for apps in the play store and doesn't need special privileges.

Anyway, I like the app. Winning against hard is not too difficult. Occasionaly it makes obvious mistakes. I never lose against it, because there is no resign button. I simply restart the game, when I get into trouble. The score is more like 12 to 200. At the beginning I was sure, that grumpy is easier than happy, now I'm not that sure anymore.

Also I got caught in some draws. There should be a possibility to offer a draw (if it is a generic 3 against 3, I'm sure nobody can force a win. Also 2 against 1 is a draw in some conditions, methinks), and if there is a 3fold repetition it should become a draw automatically. Another improvement would be the marking of the last move. And I wouldn't mind if the AI resigned if hopelessily behind (or have this optional). One thing I like is the absence of an undo-button. And the UI feels good.

Very nice!

Unfortunately, master takes too long to move on my device (samsung S3 mini), so I can't play it.

Net wins might not be a good criterion to rank the players. Somebody ho loses 400 times and wins 100 times should rank higher than somebody who got 2 losses.

Starting a new game without resigning should be possible until the player (and not the AI) makes his first move (now it can only be done before any player makes a move).