Another discription in words of how to deal with the defile/Valk situation:
That said, these abilities can occur in 3 separate ways: a) each several seconds apart, b) Valk quickly followed by defile, and c) defile quickly followed by a Valk. Someone in your raid (but really everyone) should be watching DBM timers to see what is coming next and how soon another ability follows. Based on what's called out, either stack or spread.
The first scenario is the easiest and doesn't warrant explaining.
For the second scenario, someone calls out "Valk soon, defile right after." Everyone stacks close in the middle. Valk spawns. Get a stun on it. Everyone immediately spreads back out loosely around the edges of the inner circle. Defile spawns. Nearby people avoid it. If spread properly, it should never hit a 2nd person. As for the person it targets, it's fine if it ticks once or twice on him. As you said, sometimes latency is an issue for some players. The mechanic is forgiving enough to accommodate my often 500+ms latency.
For the third scenario, someone calls out "defile soon, valk right after." Everyone spreads loosely around the edges of the inner circle. Defile targets one player and spawns. Everyone runs back toward the center in preparation for the Valk (be aware of who the defile was on and shy away from that person's quadrant within the circle, in case it grows a little more than desirable.)
As far as your specific issue goes (basically scenario 2), just make sure someone stuns the valk. It's paramount that your melee and tanks don't stack up anywhere, let alone on the valk if defile is about to spawn. Stun the valk, wait for the defile, run around to the outside of the defile in the Valk's path and resume melee dps + snars, stuns and slows when the valk reaches them. Executed perfectly with snares and stuns, you can kill a valk before it moves more than 5-8 yards. There's no rush. Avoid the defile first.