Well, if we do an attack form, it could be something like a rogue, using disruptive attacks to slow and poison the enemy. Defensive should obviosly be a tank, able to take lots of adamage, but have a slower or weaker attack. and the spellcaster could be a supportive mage or druid, having buff abilities with each forms. Like healing, the bear form's roar and tanking ability, a crocalisk form for powerful attacks and some buff spell to go with it, and an aerial form for scouting and aiding air units, since he would be melee.