[K]

The most common newbie mistake is to ignore the power of magic(spells and gems).

Strategically, it has a lot of use. Invincible armies of troops can be destroyed outright with a single battlefield enchantment, but it is even the small spells that can turn important battles. A few mages casting Falling Fires allows them to kill far more troops than their gold cost would indicate.

Using gems wisely is also of primary importance. Always look at the number of troops killed (or saved) for your investment in gems. For example, a Sceptor of Authority will kill far more troops over its lifetime than Fire Sword for the same gem cost, but for triple the gems a Wand of Wild Fire will kill more than triple the troops. A Flame Storm will kill enough troops in five castings(total five gems) to put all to shame.

Research is way of more efficiently using your gems as higher level effects become better. While the late game requires powerful mages to get to these magics, Communions, boosters, and the summons are all ways to reach these powerful magics.

In general, your overall strategy should be based on what magic you are pursuing. For example, Fire nations are wise to invest in archers(Flaming Arrows) and Fever Fetishes (requires Nature and Fire).