Toughness and toughness upgrades actually SLOW the rate at which you gain EXP. Even with upgrades, it is faster to continuously abort than it is to let them man jump like that.
This game seems like the major difference between elements is simply Fire = Money and Ice = Experience generation. If that is the case, then understanding that big numbers and spells amplified to a higher level = more experience generated, I have consistently found that Fire ends up winning on both fronts, with little reason to go ice.
This is after I have restarted the game multiple times, alternating the two elements often.
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