There is no "safe way" you can do it either way. If you're planning to get married in Canada just make sure you have a return ticket and she has strong ties to her country (Job, properties etc.,) when you cross the border. You don't have to tell the border officer your life story, just simply tell them you're visiting each other and want to be together, if they ask more questions then you can provide more information with proof of strong ties. It's not illegal to get married as a visitor, this is what I did at my spouses home country.
Now you can definitely have her come in and say that she's going to get married that they're obviously going to need proof she's NOT going to stay in Canada indefinitely. Best way to tackle this, would be just simply stating she's visiting, if they ask give them proof that she's not going to stay in Canada forever.... that's all they want, for visitors to not overstay. Once you cross the border, it it will be easier, you can apply for inland and request for visitor extension or ETA depends on where she's from.