The rockfall rain happens after the quake. The random rockfalls that are outside the massive rain are not near enough in quantity to actually kill someone unless they are standing right in them. There is just no excuse there.
While yes as you say, you can not decide "this turtle is gonna die here", but you can still see where a turtle is before he dies, or where he is going. Adapt. Also keep slows on the turtles, they aint moving that fast then. If you still have problems with the turtles, then your raid group got more problems than "rng" fights.