1. As soon as possible make an orc pit. Make your orc pit pump orc hordes. Still produce economy, but sparingly. Dont have idle time (time where nothing is being built) at your orc pit if possible. When you get a sufficient number of orcs (this number will be small) attack with them. Make sure that they dont get killed. If youre facing strong Gondorian soldiers, which you shouldnt because they cant be trained as fast as orcs, then only fight them if you will beat them with minimal casualties. If there are too many, run away and attack buildings on the way. Try to inflict maximum damage while preserving orcs. Use Eye of Sauron ASAP to give you the edge.
2. Retreat that 1st wave. Raid at random intervals but never engage them in combat with troops. At this point your enemy will have troops, at least a small army. Since your forces are spread out, not concentrated like typical Mordor forces, you will rely on geurilla warfare and hit & runs.
3. As fast as possible after your "eye" has dominion over the top and bottom of the map, get a troll and send it along with orcs to attack from the middle. This is your main army now. Youll be surprised what one troll will do. Use him to inflict as many casualties as he can before dying. Meanwhile target buildings with your orcs.Keep doing this and apply heavy pressure until your enemy dies, meanwhile steadily improve your economy.
The shock of part 1 will give you an advantage and your opponent will take at least minor damage from your raids. The reason for not attacking in part 2 of this strategy is because your enemy will expect it and your troops will not be fully able to win battles. Give him a false sense of urgency to mass troops. His economy will suffer as a result. Then as he starts to relax go in full force with that troll and kill his already-stretched economy.
You can discuss this strategy here.