In my co-op TWW3 campaign, I'm playing Azazel, and all the faction mechanics are new to me, so it's been tough going in the beginning. For one thing, I didn't figure out that there were specific cities that I could take and develop until around turn 30, so I only had my starter city and 1 decent army and one completely crap army. Recruitment has also been something I had to learn, as they recruit very differently than anyone else I've played.
Also, we're playing without mods, so it took me awhile to get my tier 4 units. On Hard difficulty, you can take an early game 20 stack up to a settlement and the auto-resolve often tells you that you are going to lose, which is a huge pain. You either have to manually fight or encircle the city and wait them out, and I try to manually fight as few battles as possible when I'm playing co-op.
Had a nice battle where I was way heavily outmanned and still won, which is somewhat rare for me on Hard or above. I actually went into the battle just wanting to kill as many of them as possible to make it possible for my other army to come in and finish them off. There was nothing really special about the battlefield except that I was going to have to fight uphill if I didn't move after the battle started. Also there were two patches of trees. So I divided my army into four groups, one large and three small. The large group was on the far right, one was on the far left, and two, which had mostly faster units, were hidden in the trees.
When the battle started, my large group marched to the top of a hill further to the left, and then we just waited because we don't have artillery available to us. The Dark Elves also waited because they had reinforcements coming, and we weren't bothering them. So when their extra units arrived, they started moving to go after the largest force on my right, but they had plenty to split off and go to the left. As soon as they got close enough on the right, my units charged downhill to meet them. On the left, once they engaged, I moved both groups of hidden units to flank and surround them. The Dark Elf units didn't care for that, were losing health quickly and started routing almost immediately. As it became clear the left side was in good shape, I started sending the faster units over to hit the right side in the rear (no comment, Brian). Soon enough I had all my forces surrounding them on that side.
Generally speaking, the problem with this strategy is that you have to make sure that you can take care of them in a hurry on your weak side or your strong side can't hold out long enough even if you put your best defensive units there. That depends on the composition of the armies, of course, but in this case our units were about evenly matched, so I was running the risk of being bogged down on the left and never getting to reinforce the right.