You wanna put your bubble in an area where you distract the enemies fire from an area that is normally a hot spot. For instance, putting a bubble in the central right hub area near B from shores of time will allow your allies to shoot at the enemies with your bubble getting in the way (like it does at a control point) while still providing an area or safe haven (pedestal rock route from B) while making enemies choose between you or the 2 general areas of focus. That's how you use a bubble.