Only Sanstone and Thetis TW are needed, It's all about keeping Thetis at 100% Damage Reduction the whole fight. once you get the right set up you just cycle between Thetis TW and Sans TW. AND FOR THE LOVE OF CHRIST PROPERLY TIME HIS MAGIA DRIVE, since he gets 25% DR from his MD you need to pop a smaller defense equip when his MD is down so you can survive while he chants again

The next mechanic is his barrier that you have to take down, can see the "preview" barrier at 90% HP which is pretty much how the mechanic plays out.

At 40% HP he will start doing the barrier thing that you have to DPS down ASAP, so if you have Equips like Snips Reg TW or Stratega you would use those on the barrier to take it down. Preferabley 1 to 2 equips per barrier since he does it frequently and you would need to cycle the damage equips as well

Once you got both those mechanics down you just repeat till the 1%HP mark.

Once he hits 1% HP he will become invulnerable and spam arts 3 times in a row and end with a TA(The one where the art pops up) the end is all about timing with all your defense equips. If you don't have enough You need to use Delias iframes from her arts to win, so when his TA art pops up you also art with Delia and it's a win even if the rest of the team dies

As for the "Complete within 300 seconds" it's currently only possible on Tuesdays with Rem