good looking god-rays that will work on iPhone?

Try using line render - use a alpha'd texture. Works quite well.

There is this system on the asset store https://www.assetstore.unity3d.com/#/content/13196 It generates light shafts based on a plane like @SAMsan suggested, it only takes away the trouble of setting up hundreds yourself and have real-time collision + multiple other options.

If you want it to follow a specific casting order you can even create a mesh (external program) and place vertices on all the spots you want a shaft to be casted, the system will use that vertex data (not the actual mesh) and create casts on given positions. This can be great for large scenes as well underwater.