To get this achievement you need to complete the quests "Information at a Price', & "A Plot Revealed" which becomes available in that order after your advancement to Warlock.
Once you complete these quests Raminus Polus will advance you to Wizard Rank.
If you need help with any of these quests, check out these links for a walkthrough.
Information at a Price:
http://www.uesp.net/wiki/Oblivion:Information_at_a_PriceA Plot Revealed:
http://www.uesp.net/wiki/Oblivion:A_Plot_Revealed