Why would you choose to stay at a place that is "borderline dangerous"? You could have cancelled for inaccurate listing info with no penalty AND get Airbnb to provide about 10% additional credit for you to be re-homed. Also.......agreed that 27-30C is uncomfortable but let's get real.......this is not anywhere near dangerously high temps. (how uncomfortable you were would highly depend on humidity levels in addition to temps)
If everything else was as described except for the A/C, then I would think it would be max 15% and only because it was during a heat warning - personally I think 10% would be more appropriate.