can a p.i for marijuana be beaten. especially if innocent

Any criminal charge can be defended, even if the accused is guilty (remember the OJ murder case?), and especially if he is innocent. That's what criminal defense lawyers do all the time. Even when they are unable to successfully defend, they often can work out a plea bargain to a lesser offense or a diversion or ACD program.

Of course it becomes very hard to successfully defend when there is a prior record, the results of the alleged illegal conduct were widely observed, there were tests run, and the evidence is definitive. So from what you said, it may be the best you can get is a plea bargain and that can be major.