Unless I go on top of the roof, it's the only to get to it without a step ladder. I don't know if the covers on these things are meant to stand the weight of a 250lb guy. Aside from falling and breaking my arse, could I do serious damage to the unit itself?
The condenser is usually made of simple sheet metal. So no, I would not recommend standing on it. You could instead use 2 ladders on either side with a platform between them.