Orcs have no magic. Warcries.

Halfogre already wrote that defense should be taken not by the main hero, but by a side hero, for the sake of the skill "protect us all" (learn to read :)). And it's worth accumulating goblins not in order to move mountains with them, but in order for the "fury of the horde" ability to hit harder (1 unit = 1 damage). For the final battle, it's better to take shamans, as their magic damage has a real effect (+ if you're lucky with the "silence" skill, it's even better), unlike traps. Besides, it's not a given that the enemy hero will go down before the orcish hero (if they're not very smart :).
Regarding leveling up, yes, goblins are used as a living shield for the centaurs + traps. I also take warmongers with me, but their function is only to occasionally finish off enemies.