Here is the full list of all 16 Grandma's Footsteps achievements.
Take your first footstep
Complete Level 1
Complete Level 2
Complete Level 3
Complete Level 4
Complete Level 5
Peace at last
Walk 1 Mile (5280 Footsteps)
Bravely run away!
Unlock Major Tom
Unlock Fishface
Unlock Jack In The Box
Unlock The Jonathan
Unlock the Old Oaf
Get shot by Grandma 1000 times
Take 138435 footsteps