If you have Zhongli, it’s piece of cake— you shield up, then use whichever DPS character you want to brute force through the Ruin Guard. Shouldn’t take longer than 10 seconds.
If you don’t have Zhongli, it’s slightly more complicated. You’ll need to learn the Ruin Guard’s attack patterns, and be ready to dodge them. You should also probably have a bow character to hit the Ruin Guard in its weak spot (eye), which will give you a small window of opportunity to get some free attacks in.