So, if we're going to jump on this "Nerf Wizards!" bandwagon, let's see some other nerfs.
First of all, all powers that deal damage upon entering the zone should be limited to once per turn. Note that this should apply not just to Wizard powers, but to powers in general. I'm not going to go through and itemize them however.
Furnace of Sand also needs to be nerfed to a static value, probably 5 + Intelligence modifier.
Stonewrack can serve as a passable exception to the once per turn rule as long as we modify the text to read "Whenever a creature that is not prone enters or moves within the zone-" that way, the first time the target suffers damage, it falls prone, which would mean it's immune to suffering the damage again while prone. Due to its smaller area and immobility, we can accept the Prone element, so it makes sense to give the damage a Cloudkill esque level, at a flat 10+Intelligence modifier.
If going through with all these nerfs, I strongly recommend changing Summon Marilith to instead make melee attacks against all enemies within reach, rather than the intense damage of multiple hits. Possibly add a rider that increases the Marilith's defenses for every hit (in order to compensate for the fragility of summons).