Some primordials are more powerful than others, as are some deities.There were several primordials stated in 4e. However, it is not true that they had to gang up to defeat them one at a time. In 4e, both Bane and Io are noted to have slain a primordial one on one. However, it seems that the common approach was to gang up. For example, Maul-Tar was imprisoned by the combined might or Moradin, Bahamut, and Pelor I believe.
she should have stats similiar to Tiamat, that means CR 25-30
Long story short: The stats for Meg are laughably weak for how she's described in the Drizzt novels. However she should still be some of the weaker primordials as far as immortal cosmic entities go.