The Lord of the Rings: Tactics

Black Gate/Mt. Doom:
Get Stealth Of The Shire 1 and Courage Of Sam 3.
Use it, and you can block all attacks on Frodo and Sam.
Note: It doesn't work in multiplayer mode.

Gaining experience:
While playing throughout the game, complete all of the extra battles.
This will increase all of your heroes level, which will be usefull
in the harder battle later.

Host Of Mordor:
When playing as Host Of Mordor, always do the extras with Sauron and
The Witch-King to increase their stats for the last level.