From the datafiles:
Rodhzidron is just a higher-level Rothgoroth.
A 40 level Rodh is exactly the same as a 40 level Roth, except
1) it costs much more essence and
2) does not need a lot of experience to get to that point.
I don't have the files handy, but IIRS:
Roth has base-level 34, Rodh has base-level 40.
To make a Rodh, you need +3 more to battle shaping, which makes your new Roths level 36.
Not a big diference, if you ask me - some 4d12 of damage, when the roth would already be doing around 40d12.