That depends on what characters you have.
For Zhongli owners:
- Activate Zhongli’s Jade Shield.
- Switch to your main DPS character and proceed to royally fuck up the Ruin Guard.
For everyone else:
- Keep your distance and dodge well.
- Use a bow character, and fire two shots into the Ruin Guard’s weak spot (its eye or the middle of its back)
- Switch to your main DPS and go ham on the Ruin Guard.
- If it gets back up before you can kill it, go back to step #1 and repeat.