It is possible to predict what your opponent may use after you play a lot of games and notice the common build orders most players use. At Armageddon, you can expect most players to send a giant. An even better player will scout your defenses, exploit your weakness, and send based on your weakness. For example, if they see you have a lot of archers but lack good anti-ground, then the better player will most likely send a giant against you (giants are strong against archers).
@jollow250, not always. DPS and performance are very important characteristics for towers. For example, archers do poorly against high-HP mobs or against ground mobs in general -- the TNT tower does better against general ground mobs than the archer but doesn't target air units. Against mobs with regen or if they have regen support, high DPS is even more important than just "any damage."