Have you tried a weed wacker? At so tall, I would bring it down in steps, maybe a third of the remainder at a time. When it's down to about 6", you can try the mower on its highest setting.

I agree with dovetonsils. For awhile I kept my St Augustine unmowed. It grows to 32 inches and stops growing. When the city threatened to fine me for keeping it higher than 18 inches I brought it down with a string trimmer. When I sold the house I was keeping it at 12 inches tall. The new owners did not listen to my suggestion of bringing it down with the string trimmer and quickly broke their lawn mower on it.