The Microsoft Store now offers the StarCraft II Campaign Collection for Windows, which can be part of an official installation.
Do you need instructions on how to manually download and add for offline use? starcraft 2 offline installer
Crucial Note: Do not stop downloading when the launcher says "Optimal." You must let it finish completely to ensure all campaign assets, cinematics, and offline maps are stored locally. The Microsoft Store now offers the StarCraft II
Here's the step-by-step process for creating your own portable offline installer: starcraft 2 offline installer