I've had a few questions about this, so I'll take a minute to explain some specifics about AT and RNG rules. AT and RNG can never be less than 0, not even temporarily. If you attempt an effect that will lower AT or RNG below zero, that it will take partial effect only. For example, if I have 10 AT, and I get hit for a -20 AT effect, the effect will become a -10 AT effect for purposes of effecting me. You will see a red '10' appear over the unit instead of the expected 20.
One side effect to this is that a temporary effect can protect your AT or RNG from a permanent one. If I use Mayleen to drop a target's attack to 0 for the turn, any AT draining attacks, permanent or temporary, will read that AT cannot be dropped below 0, and thus have a draining value of 0.
Oh, and I'm pretty sure that applies to DF too, but as there is no effect in the game that drains permanent DF, as opposed to just setting DF=0, I don't think it'll come up. There also isn't any power that temporarily sets DF=0, although there are two that give -10 DF for one turn.