Is the place you need to save just 1 step before the battle encounter, or do you need to save earlier on for a legendary that you will see in the game but not encounter in battle? (e.g. Latias/ Latios in Dreamyard in B2W2; you need to chase them around).
If you chase them till they stop, then you save right in fron tof them, you can Soft Reset until you get a shiny. (Soft Reset is Pressing L+R+START+SELECT at the same time)
You save one step before the legendary and it will still work and keep reseting until you find one.