Guide how to do it right:
1) Ally with Gondor to ceasefire with Rohan;
2) Ally with Rohan;
3) Ally with Eriadot and Elves to save your back;
4) Take rebel settlements and build up economy;
5) Be “good guy Greg” till Sauron emerges (around turn 50);
6) From monies and units you accumulated during years of peace and prosperity create two strong full-stacks (you will need ballistas to quickly take walls, Dunlendings as line infantry and Urukhai riders as mass swordsmen, some archers, wargs and elite is nice addition).
7) When Sauron emerges call treacherous invasion to Edoras, but don’t go for Edoras.
8) Use 1st stack to take Helm’s Deep (and the castle on its way) and 2nd to take Gineard and all Western lands of Rohan.*
9) Now half of Rohan is yours, you have allies behind your back and your fellow evil guys have taken Edoras (you can purchase it for yourself now if you want).
……
?)
Profit.
*) EDIT: make sure you have diplomat to cancel alliance with Rohan at the right moment. Maybe it can be canceled by joining invasion but I'm not sure about it.