Fast Total Destruction:
(This cheat is only for an un-updated version of Battle Realms which means unpatched.) This is quite easy, all you gotta do is to find an opponent's building and press ctrl+D. This cheat will work when you see the building collapse immediately.

Duplicated Brothers:
This cheat only works in the Lotus campaign. Do the following:

Build a Crypt of Brothers as soon as possible.
Build three Watchtowers and summon three Brothers.
Put the Brothers in the Watchtowers.
Destroy the Crypt of Brothers using CTRL+D.
Build another Crypt of Brothers
Summon the three brothers again. Now you have six Brothers.

Easy Yin Points for Serpent and Lotus Clans:
In the Serpent Clan, use Ronins and equip them with Yin Blade. Then, force attack them to trees while using Yin Blade.

In the Lotus Clan, summon the 2 Brothers Lythis and Tausil, then have them attack water (force attack them into a very shallow spot in the water or other similar place and also a little deeper on the water). While they attack, you continuously yield Yin Points.