Assemble your forces from the start from Minas Tirith, including Boromir and Denethor and attack east Osgiliath right away on turn 1 or 2. Should be easy prey. Afterwards, ignore the rebel settlements around there, until Mordor takes it. Buy some barracks in Minas Tirith and Dol Amroth later, but put the rest of all your money in your economy. assemble a force in the west right away to take Annulond. Afterwards, send it to Dol Amroth where you can build up a force too. Meanwhile, have about half a stack run around Ithilien to intercept any incoming Mordor stacks and send a few reserves to make it possible to capture the settlements Mordor captures. Be very eager and agressive in attacking Mordor stacks, else they merge and become full stacks, which will be harder to deal with.
While you build up a force in Dol Amroth and keep building your economy, expect some Harad troops near Linhir near turn 15/20. You won't need many troops there first to stop them, just a few generals and a bit of support. After some more settlements are yours, like Henneth Annun, Cair Andros, Druedan village etc. carefully start to build up a bit of an army around Henneth Annun. Block the bridge west of Henneth Annun with it and see if Denethor has a shot at blitzing Minas Morgul. It has a garrison script, but plenty of small armies usually run around the place. Attack an army outside the walls to dodge the garrison script. Take Minas Morgul and then push on to Cirith Ungol. Once you've taken it in a similar way, sell the settlement to another faction while getting an alliance with it (so that faction won't attack you). I gave it to Gundabad myself; That way Mordor won't attack you from the east anymore. With about half a stack on the bridge near Henneth Annun, Mordor can't enter your lands anymore.
Meanwhile, you prepared a force in Dol Amroth. If you're ready to sail with about a fullstack on turn 21, you're on the same scedule as me. The sooner the better as Harad seriously starts to build up between turn 30 and 40. Sail for Umbar and take it asap. Dodge the garrison script if you can, otherwise just accept it. Take Vatavea right away too.
Meanwhile, Denethor is free to attack south with the assembled army he has. The troops you had near Linhir can cross the Anduin as well and go for Harondor provinces. If you're in time, Harad still shouldn't have too many troops around there, especially if you just took Umbar.
With these 3 forces on the move, your economy will be strained, but settlements will be easy to take, which will quickly "repair" your economy and will soon allow for even more troops to be recruited. The key is to keep moving and use a spy or 2 to open gates. Note that the only 2 settlements in Harad territory that have a garrison script are Umbar and Gobel Ancalimon; the rest can safely be opened by a spy or just siege equipment. If you keep moving, Harad's armies can't catch up with you. I managed to kill Harad off this way at turn 53.
edit: I've reloaded a few times only because I had a spy that died on me and once when Boromir caught a catapult shot in the face.