The answer is definitely the lightmap resolution but here’s a slide show that I made to demonstrate what is actually causing the problem and a solution:
Short answer is; Your “floor mesh” is just too large for a low res lightmap. Scale the mesh down to 1/4 the size and then build it out of 16 meshes instead (4x4). You should always build your levels out of several smaller static meshes instead of scaling meshes to fill the entire scene. As you scale a mesh up, it requires a higher lightmap resolution to give you the same visual fidelity. The higher lightmap resolution will affect performance on mobile more than instancing your floor mesh will.
I’d like to point out one more thing these answers are not covering. 64 resolution lightmap will NOT produce wavy shadows! The problem is as TorQueMoD pointed out the floor. If you’re using landscape, be sure to set ITS light resolution to something higher, like 8 (8 means 64 because in landscape it’s 8x8, you can read it in the tooltip). Just select it in the viewport and edit it in its details.