Tannins are a family of compounds with several different structures and properties. Some of them will be degraded at higher temperature while other will not. While try to preserve bioactive compounds from plants, it is recommended to operate at 50°C maximum.
You can also check the effect of various temperatures on the tannin content of your plant, then choose the best temperature!
I agree with the wonderful suggestions, you can also consider extracting with less polar organic solvents or methanol-water mixture in varying proportion.