Water it as often as you can while the heat wave continues - twice a day may be needed. Mulch would be good - whatever mulch you have handy, pine bark is good. If you can rig up some shade during the hottest part of the day, it will be grateful.

Hibiscus normally produce yellow leaves whenever there is a change in conditions or they are stressed. Pick off the yellow ones and it will have nice new leaves within a few weeks (assuming it doesn't dry out and die in the meantime!)

Quite frankly I'd be shocked that the heat would be hurting your hibiscus. If I'm translating your temperature to F. right, your 'heat problem' at 30 is a cold front for us, where we grow hibiscus that become small trees.

Water, however, is a real problem; they're thirsty plants. Could you maybe just stick the whole pot into a bucket of water? We grow these guys in our wet gumbo soil and I have yet to see a hibiscus die of overwatering.

Though the top of the plant doesn't mind extreme high temps, the roots are far less tolerant of extreme heat. 30* C. = 86* F., still well within the desirable range for hibiscus, but add in the solar gain factor from a dark colored pot or a pot that sits on a heat absorbing material, & you may have roots that are unable to metabolize &/or move water. It would be helpful to shade the container or find another method of reducing root temps if the plant is in full sun.

Al