Game-Breaker: putting firewarriors into balloons and casting Magic Shield on them makes them completely invulnerable. Bonus points if you position them around the enemy shaman's reincarnation circle: each time she reincarnates, the firewarriors in the balloons will target her, killing over and over as long as she has a single follower.
Cleverly arranging priests in guard towers, and then putting guard towers with firewarriors slightly behind them to protect the priests can lead to virtually unbreachable defenses. The approaching enemy army will be converted by the priests (and thus add more members to your tribe), while the enemy priests and enemy shaman will be picked off by the firewarriors. Put your shaman in one of those towers to cast Magic Shield on her troops, or cast Lightning (for the enemy shaman), hypnotize (if you're impatient) or Swarm (if the enemy priests are persistent) will make things even more difficult for the enemy attacking. Also construct a temple or firewarrior training hut near the towers and you can easily train the new converts into more priests and firewarriors to man more towers which you can build. Over time the enemy tribe wastes their energy and numbers trying to get through, while you get stronger with more converted members and more Mana from all the times you've slain the enemy Shaman. About the only way to break through this deadlock is if the enemy Shaman uses Volcano, Earthquake or Tornado, but this is often a one-use spell, and after the destruction caused by it, you can just repair your defenses and continue.
Goddamn Bats: The Swarm spell (see Bee Afraid on the main page). It doesn't do a lot of damage, but will make the affected units impossible to command possibly making them run deep inside enemy territory or quite possible screw that rush with Preachers and Warriors.
Enemy spies. They have the ability to disguise themselves as your braves, infiltrate your base, set your buildings on fire and cause mayhem and chaos.
Enemy firewarriors vs. your Shaman, especially if they're a) in large groups, b) in guard towers, c) on high ground or d) all of the above. They become Demonic Spiders if you send your Shaman to attack them in a boat or balloon, as it only takes two shots to kill her in those situations.
In multiplayer, the "uber" trick can make the targeted unit's health variable overflow, making it's health bar go off the screen.
Similarly in multiplayer, there is a trick with the Landbridge spell that makes a small area of the ground go to it's maximum possible height. Casting Flatten onto the area afterwards will create a VERY high plateau which clever players can exploit for gaining the advantage via high ground.
Using hypnotise on enemies and then sending them to train changes them to your side permanently, it also also allows you to gain more the the unit cap (200 followers) and potentially crash the game. ( See Arbitrary Headcount Limit)
For those wondering, "Kai!" and "Ka!" are (respectively) the selection acknowledgement and order acknowledgement given by the shaman unit, while the "Dong" the above troper refers to is a warning drum that is sounded when enemy units approach a manned guard tower...
Also "Shoka!", when said by an enemy shaman. *lightning fries your shaman*
"Oonaga!", followed by the sound of your followers screaming and running as they're attacked by swarms of bees.
Porting Disaster: The Playstation version, which lacks the music that cues when a fight starts and there's an incorrect color assignment between the Matak and Chamara tribes.
That One Level: Level 15: Incarcerated. As the name of the level suggests, the shaman is locked up and you have to free her, without her spells, of course. Within a 7 and a half minute time limit. Might be considered Guide Dang It! too, as the key of the solution is training various skills in exact amounts and then not killing them during the landing.