Defeating Large Groups of Enemies
Have a lot of defensive structures in your base and teleport the opponents to your base. The towers should kill the enemies before they can do much damage. The Convert spell will also work, because they are free men.
Ring Heroes
All maps in Skirmish and Multiplayer mode have Gollum cloaked on it. When you find Gollum, kill him and you will get the ring. Take it back to your fortress and have the troop that is carrying the ring enter the fortress in order to recruit a Ring Hero for 9,000 resources. The Ring Hero does not use any command points. Men, Elves, and Dwarves can recruit Galadriel. Mordor, Isengard, and Goblins can recruit Sauron. The easiest way to get the ring is to let an enemy find Gollum then kill them after they obtain the ring. The only way to stop them before they reach their base is to chase them down with a mounted unit, have a created hero teleport into their path, or to summon allies into their path using a power.
Grey Havens
Start at the bottom-most part. Build as many resource collectors as possible. Then, capture the Shipright. Get Builders to build Wall Hubs where the Cave Trolls lair is found (kill it first). Build a wall, then build units and an upgrade structure. Get your units upgraded as much as possible. Put some by the wall to defend it. Send the rest to the Shipright. Build the number of transports needed. Put the units by the Shipright onto the transports. Send the transports to the enemy camp. Optionally, split your units up and surround the camp. Destroy all the structures except for the fortress. Finally, destroy the fortress.
General Strategies
Do not just rush your opponent’s territory. Either send in a camo capable unit or a very cheap, weak unit such as Orc Warriers. Check out your opponent’s base before attacking with stronger units. When attacking, use melee soldiers to attack buildings and use Bowmen and Cavalry on enemy units.