The squads can have different specializations, depending on the team leader. You might assign a healing character to stand towards the back of the melee, casting spells and protected by allies with long-range attacks, while a tough soldier-type would have hard-hitting squaddies to help him smash through the middle of the enemy's defences.
Making progress through a level generally involves hacking your way to the next monster generator, where you can summon new allies to replace any that didn't survive the journey. Some of the best levels pit your team against an evenly matched group of opponents, with generators of their own which you must commandeer to force the enemy back towards the crystal that marks the ultimate victory point. And they'll be attempting to do the same to you.
With a close-up view of the battlefield and limited stylus controls, the game isn't really designed for intricate strategies. In fact it has some strange quirks that are probably designed to discourage players from treating it as a traditional RTS - for example, groups of units are instantly deselected after you give them an order, and are hard to find again once they wade into battle.