Try this formation:
(1 star, Major, Private, 5 star, captain, 3 star, Private, Colonel, Private.)
(Private, Lt.Col, 4 star, 2 star, Private, Private, Spy, Private, Spy)
( 2nd Lt., blank, blank, Private, blank, Flag, blank, blank,blank) I learned this technique from a friend.
This is a defensive formation but can also be an offensive formation depending on your situation. On the left flank you may notice that the only defense it has is the 4 star. That's because your focus is to either eliminate his/her high rank pieces. Let him/her eat your pieces on the right side of the flank because once he/she eats one of the privates then eat his/her piece with the spy. He/She won't suspect that flank has a double spy so he'll try to avoid the spy and eat the other private then if he/she does pawn it with the other spy. I'm sure that if he does those moves you would've eaten one of his 1-5 star generals. Then I'm sure he/she has a spy on that corner but once you've eaten it then you can pawn his middle flank(if he/she doesn't have a spy) and his right flank with your 5-star and 3-star and you can get your flag across and you've won.
Note: It always depends on you on how you will use this formation.
The flaw: If he/she pawns your left flank then your only option is to get your flag across in the middle with the 5-star and 3-star and the spies.
Tip(unrelated to the formation): Your 5 star will most likely be always the bait for the spy. How to avoid?
Notice his/her pieces when they start moving closer to the 5-star(or any other high ranking pieces) which is always either a private or a spy but 95% sure it's a spy. Once you've identified his/her spy you can now avoid it and most likely his/her other spy is in the other flanks.
Hope this will be effective to you. Good luck and happy playing.