I found this on the comments section of 3++ is the new black and thought I would share it. It was in a discussion of how to beat tau and chaos daemons came up
4 Level 3 Heralds on Disks in a Screamer unit, with Fatey along to re-roll failed Grimoire checks. The Heralds go for the 4+ invulnerable power for their squad, then the Grimoire pushes it down to 2++. With Tzeentch's "re-roll 1s" inherent ability, if it gets both the psychic power and the Grimoire test it's basically unkillable on that turn. With jetbike movement, it's shooting/assaulting the Tau on Turn 2.