Re. the Path Functionality, I couldn’t give you complete directions myself (I’ve shied away from MSP 2013 and have never tested it), but the office blog gives instructions here: https://blogs.office.com/2012/09/26/introducing-task-path/ . The driving path for your phase completion task is what you want.
The task path tool seems pretty rudimentary, but still much better than anything available out of the box in prior releases. It’s one reason why we held off porting our own logic filter tool to 2013 for the longest time.
The way I understand the SDLC waterfall, you would model a decision gate (a merge point in the schedule network) between each phase – with no other inter-phase links. Then of course the previous concerns go away. I sense your issue may be a little more complicated….
Please confirm whether or not the task path works for you.