At the start of the game you are only given songs to "move forward" or "attack", but as the game progresses you will unlock further songs. Due to the way you command your army, there is a delay before they act on your instructions. So rather than relying on quick reflexes, you will need to learn to predict what the enemy is planning to do. To compensate for this, the larger enemies telegraph their attacks in advance, giving you a chance to react in time.
There are three main types of missions in the game: hunting missions that allow you kill animals to gather resources, combat missions where you fight enemy "zigotons" and progress the story, and boss missions where you fight against a single larger creature. If you get stuck on the story missions, hunting and boss missions can be repeated to gather more resources and items. None of the missions are particularly long, so can easily be finished in a single sitting, which is good for a game on a handheld.
Between missions you will be given the opportunity to upgrade your army. As you move through the game you will unlock new types of units, which can be purchased using the money and and two resources. Each unit type requires a different pair of resource classes, and using rarer resources from a class will result in better units.
While you can easily find most common resources in hunting missions, the rarer resources are usually only dropped by bosses. Also, there are a number of mini games that can be played that effectively let you convert common resources into rarer ones.
While most of what I've said has been positive, my main criticism of the game is its uneven difficulty curve. There are a few storyline missions that could present problems and block your progress in the game. If you are not sure what you need to do to progress this can be quite frustrating. In fact, when I first started playing the game I got stuck on one of these levels and gave up in frustration, and only finished the game after starting over a few years later. If you do run into a problem level, I strongly suggest looking for a guide that has enough information to unblock you since the later parts of the game are quite fun.