If you are like me and thought that this was almost impossible since you barely ever stumble without dying, let alone 20 times in a row.
Fear not as you don't need tons of Anks to keep you resurrecting since I found a way you can trigger the stumble safely without dying
Of course, this must be done in the running sections of the game, not on the bike or wallrunning.
Before starting this, you might want to buy 20 of the boots.. thingy that help you recover fast so you can sometimes get 2 stumble in a row and also keep track on how much you need left.
So, when running there are always 3 lanes in wich you run, Left, Center, Right and very often an obstacle comes in your way, some must be jumped or avoided BUT any obstacle that block 1 or 2 lanes, for example this:
You can actually SLIDE to the side just as you are passing it to crash against it and Lara will stumble and you will continue running.
You can trigger this with any Statue or as you're passing a door, obstacle, root, tree, Etc. At one point you even pass next to a big wall and you can get 2-3 stumbles if you do it quickly enough.
Just make sure that after it, there is running space and not a hole or other unavoidable death.
I got my 20th stumble after hitting a statue during the boss fight.
*Thanks to Illum1na for telling me how to takes in-game photos for the game :D
Hope this helps you as once I figure this out, the achievement was really easy :D