Prepping has never been complicated to me, really. T1 handles the balance, prepping finishes it up.

In that example, kelver vs fireball, without a charge, it may cause some damage, but the kelver will probably diminish most. With each charge the fireball will get stronger. one charge, it can probably burn the kelver, and go from there. two breaks through, type of deal.

Now, if its magical kevlar, and it has charges, it can counteract the effect. 

we can, however, leave it up to combatants and hosts, as far as these things go. that seems to work well for us.