2 things here thought could be clarified a little. 1 being the aspect of "living esclusivly out of Canada, and the other being proof of commmon law etc.
First off there is no particular benefit in living soley outside of Canada when applying, it actually would be considered a detriment in a sense. The reason is that people may make an application with no desire to actually live in Canada, and get the PR as an insurance policy should things go wrong in another country (i.e lebanon). However there are ways to mitigate the disadvantage, and demonstrate reasonably that you do intend to live in Canada when sponsorship comes through. As for proof, you don't necesarrily need to acquire residence, it might not be a bad idea to write a letter stating that you lived "exclusively outside of Canada" for a period of 18 months as a test period to try out the Dominican Republic, and to grow your relationship, you did not acquire residency as you were not sure exactly how long you would be saying, and you were advised by authorities that you were ok to stay, so long as you paid a fine on the way out (in my case, that's what the DR embassy told me)and that you are now ready to go back to Canada. You can substantiate this with a tax return stating that you are not a resident, passport stamps showing your entries and exits, and other documentation, rent receipts, phone bills, testimonials etc. A lot depends on the officer, so if they are not convinced, ask what other evidence they might need - they do need to give you a chance to respond to their demands, and they do need to be reasonable, if you have evidence that documents a large chunk of your stay, and they want more they are not being reasonable, you can state this.
As for common law, what they want to see is interdependance, proof of time living together in a romantic sense, and merged finances to a degree. Rent contracts, affidavits (testimonials), phone bills, photographs bank accounts etc. are all proof of this. If you can reasonably substantiate this, and they still want more, again they have to give you the opportunity to do so, and cannot flat out reject you. If they just say "more evidence" ask for specifics. If you still feel that your application isn't as strong as it could be, or that they are doubting your application unreasonably - it may be a good time to get a good immigration lawyer or consultant on retainer. You can have them sign the "use of a representative form", and it can send a message. Basically, if you do all in your power to meet the requirements you should be ok, if they are unreasonable, you should tell them directly, and that based on consultations with counsel, if refused you will appeal. If you seem/are informed of your rights, their duties as described by IRPA, they may not be as inclided to just stamp declined, and will take you more seriously.
Again, honest, straight forward, proactive and patient. And the key with dealing with an application that hits a few bumps them asking for clarification - or you asking them what else it is you can do, if you get specifics better. As much as people will have you believe that with immigration you are guilty until proven innocent - you still have a right to a fair trial.
Now, if you are seriously lacking material evidence, and are common law and really want to spend the rest of your life together, marriage might not be a bad idea.