Tbh the Warsphinx doesn't really shine vs VC and OG. It is made to deal with elite units, who generally rely on S5 and 6 to kill stuff. For this reason it works best against elves, WoC and DoC, as they have very little (witch elves aside) as anobvious counter, and also very little high strength non magic shooting.
Casket of souls seems a good bet against those OG war machines. Isn't the poisoned shooting rather short ranged?