Well, phase 2 was the biggest problem for us because of the valks mixed with defile. We saved everyone except 1 person. It's not easy to get everyone to spread then stack again and again. The key point to keep in mind is this: Since the valks take the player to the CLOSEST edge, then you can control the direction of the valks. If possible, try to have everyone stack on the same side to have the valks travel in the same direction and close to each other so you can nuke them together.
However, it's not easy. Especially if you spread for defile and 1 second after the defile you get a wave of valks. This means you won't have enough time to stack for the valks. UNLESS you want to do what for example, Paragon does and stay stacked. If that's the case, then whoever gets defile simply runs away and the rest run away from that person. However, it's a risky strat but if all 25 are well aware of who defile is on and react fast, then that's the more efficient way to go. If you are able to pull this off, then everyone can stay stacked on LK/MT which means:
1) Infest is easier to handle since everyone will always be in-range.
2) Everyone will always be stacked properly for valks which means you can control their direction and AOE Nuke/Slow them altogether.
3) More dps on LK since you're always stacked. You can even have the MT move LK along with the valks so that you can AOE LK as well when DPSing the valks.
Another thing you might want to consider before you decide to do this is if you have players with high MS in the raid. If defile is on them, they might react slower, causing a wipe. Well, I'd definitely love to do this as it looks professional and neater than having everyone spread(That's just messy mennn!) but I don't think it can work with us since some of us still lag/dc and such. Maybe practice on normal mode first...ah well.
Good luck.