Using Hydroguard/bennies, you said you're using bennies, so that helps keep the rot away at higher temps. In my experience I noticed at later in growth with more roots you are less likely to succumb to root rot than earlier less root growth so you'll prolly be fine since you made it this far. 77 isn't really that high since you're running bennies. I have done a full grow at mid 70's with no issues with hydroguard. The frozen water bottles works great for temporary, temps climb back up pretty fast after the ice melts, but you can get it down really low with them. I change res weekly too so that helped in my case. Just keep tabs and don't get lazy for too long.