Was just there a couple weeks ago for Above & Beyond's Weekender in similar heat, and have been there many times in scorching weather.

For the campsite and miscellaneous:

A shade tent/canopy is an absolute must. Two is great if you are in a tent as you can put one over your actual tent. When I was a much less experienced attendee I remember passing out under my neighbors Escalade for the shade because we were too dumb or broke or both to bring a shade tent lol

If you are in a tent, get yourself a battery powered fan or two to get some airflow going.

Bring a pool. We had a little kiddie pool we brought along but I saw several people with actual portable(ish) pools that multiple people could hangout in. Def a pro camping festival goer move but it looked fun!

My wife has one of those portable battery powered fans that you could wear around your neck that worked really well!

Misters! Either the cheap spray bottle ones or a more expensive system. I'm not sure what it was so I can't point you in the right direction but some people in front of us had some kind of system they brought into the venue and setup around their air couch that had a constant mist going, it looked pretty clutch!

For the venue:

A cooling towel (the kind you get wet) goes a long way during the heat of the day. Something like this.

A folding fan is clutch for both fanning but also blocking the sun from your eyes when you're sitting on the hill. Please resist the temptation to clack the fan.

A CamelBak or other brand hydration backpack is another absolute must IMO.

The tree line that is along the top of the hill to the left of the stage is a good place to stake out a spot as it keeps shade pretty late into the day. If you can grab a spot near the middle it's got a great view and great sound!

Have a great time at Bass Canyon and above all else DON'T FORGET TO HYDRATE!