Spiral Abyss Floor 8-2 having trouble getting a single star

As Ruin Guards have 70% base physical resistance, Superconduct only reduces that to 30%, which is still a pretty high value given that most other mobs only have 30% or 10% physical resistance.

In addition, Ruin Guards' elemental resistances are only 10%, so you want to focus on that.

Thus, you still do not want to be hitting them with physical attacks. Focus on elemental ones, with characters that hit with elemental attacks as a default. Most of the non-Anemo 5 stars are able to do this, eg. Diluc, Klee, Keqing, Mona. Also you can use Xiangling's or Fischl's E to help with additional damage.

Just put one DPS and one Support DPS on each team, along with a Support/Healer with Thrilling Tales of Dragon Slayer (+48% damage for 10s every 20s on character switch) and you should get 3 stars easily - I did this with half of my characters having unlevelled artifacts.

Also it should be said that you should group the Hilichurls together with the Ruin Guard as much as possible, so you don't have to run around trying to defeat them after bringing down the Ruin Guard.

Its worth noting that theres an orb in the hall that keeps adding a shield on the ruin guardian.

Removing it with the hardest counter element helps (Ice -> fire) (Fire -> water) before burning any higher damage abilities (Barbara for fireshield, easy)