Exactly why you should use Toxicroak and Sucker Punch!
Believe me, on normal difficulty with a few Bulk Ups I KO'd 5/6 of her Pokemon. Only reason I was not able to sweep her entire team was because I used Sucker Punch on her first 5 Pokemon and ran out of PP for it (it never failed mind you, her Pokemon always tried to attack my Toxicroak). Her last Pokemon outsped my Toxicroak and fainted it but I still had 5 Pokemon in the back. Toxicroak shines in this gym, you will definitely want to use it in conjunction with Granbull.