It is scorching hot all across the USA, and hibiscus along the Gulf Coast is are suffering.
Plants with about 1/2 day shade, mulch, and water are doing fine. Hibiscus exposed to direct sun all day are not doing as well.
So, provide midday shade, plenty of space between plants for air circulation, then apply mulch and keep the plants watered on a regular basis.
BTW, providing room between plants also reduces spread of insects, improves branching & blooming, and helps with mosquito control by removing some of their hiding places.