I'm making this thread to help tanks understand the Dark Shaman encounter better, namely Ashen Wall and Foul Geyser. You don't really have a ton of control over other abilities aside from the starting point of Foul Stream. In the case of the two aforementioned abilities, you are directly involved with how they spawn. Let's start with Foul Geyser, because it happens first.
When Wavebinder Kardris casts Foul Geyser it has a 2-3 second (not 100% sure) cast time, at the end of which a channel begins and adds begin to spawn at your location. This not only gives other raid members enough time to get away from you, but it gives you enough time to drop the slimes in a decent position. Note that the outer portion of the Valley of Strength basically has a form of building - opening - buidling - opening, etc. Use the terrain to your advantage here - dump the adds into one of those nooks, or dance with the spawn inside one of the buildings. This not only clusters them, but in many cases bottlenecks them. This allows ranged to kill them very easily, and once tanks get the general idea, almost nullifies the worst part about the ability.
Ashen Wall is very similar to Foul Geyser in that proper placement can effectively negate the ability. Whenever you're tanking Earthbreaker Haromm, note that he always uses this ability at 50% health, and it almost always follows a Foul Stream thereafter. So, whenever you notice one of these tells, take Haromm off to a far corner, or even better - into one of the buildings. Always remember to face the boss and have your back to whichever wall is closest (or into the bulding, if you chose that route). Since the wall always spawns from your left to right at the end of Haromm's cast (as in, it's perpendicular to him), you can easily control where the walls spawn. If the walls are nowhere to be seen in the middle, it'll keep the middle of the zone clear for movement.
I hope word of this spreads, because I think we can all imagine how much fun this encounter will be in LFR tomorrow if tanks aren't trying.
TLR - Use nooks and buildings for dealing with Foul Geyser/Ashen Wall, watch less raid members die, kill boss, get purples.