Every map in skirmish or multi-player mode has 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. Your Ring Hero uses no 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.
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.