Found a solution worked first shot hehe.
From YouTube.
Method 2: 1.After you see "savedata corrupted" message, don't press anything, turn off your 3DS. 2.Run steelhax installer
Once 3.run the game.
Good luck works a charm.
Hmm I've redone the steps so many time I lost count steelhax seems to be working correctly as i press a and then a chose my firmware ver
It downloads its thing hold start press x to exit.
Start sub war and get currupt save every time. I must be doing something wrong as with op.