Putting this here in case anyone needs: Challenge Mode BIS spreadsheet. Should be basically the same as Proving Grounds since gear is downscaled the same way.
Since I'm not an alchemist, and my server is so tiny that nobody ever sells DM trinkets, I went with Searing Words and Bottle as my trinkets. There was a post from a while ago discussing CM trinkets and both of those and the Relic are quite close in value.
I ran as SV with Fervor/Blink/Barrage. Pretty sure this would be the best setup for either spec -- TotH loses a lot of its value without our 2-pc, and both DB and AMoC will not get full uptime. Binding Shot to stun the banana tossers, and Posthaste to get to new mobs faster (for amber placement). Got to wave 24 on the first try and 47 on the 4th. Haven't been back since, but I keep meaning to -- just wish they didn't make it so long.
For a strategy, I found it best to kill the banana tossers first, always. Their projectiles give a 75% debuff. It's pretty easy to avoid, but not so easy to avoid and pay attention to optimal amber placement at the same time.
Gaming the ambers is really the largest part of this. The amber puts a 50% damage taken debuff on the target and also prevents the shield mogus from putting up their shield. You usually don't want the amber to hit something that's almost dead, because that's a huge waste. I also pooled my focus for ambers. If you start with a full focus bar you can bring pretty much anything from 100% to 0 before the amber wears off.
You can also game the healers a little bit. They always cast their heal on the lowest health mob in the room (so just roll your eyes and laugh if you manage to get them to cast it on a virmen), and if the mob dies then the heal fails to cast. By abusing this I almost never even needed to interrupt.
My guide on the previous page seems SO outdated now... SV all the way, folks! Getting yourself a good set of gear like Musca suggests can get you a long way, too. I prefer Fervor/Blink/GT myself. Struggling with wave 30 to pretty easily reaching 76; it all adds up!