It appears you don't actually need 7 targets lined up perfectly in your Coalescence, but simply to hit 7 unique/individual targets while using your Coalescence. This makes the achievement a lot easier than it seems and you should be able to unlock it through natural gameplay.
Some tips: 1) allies might need to be damaged (not full health) in order for your ultimate to heal them and count towards the 7 targets you need; 2) Moira is vulnerable while ulting, so flank and use the high ground; 3) hit everyone you can while ulting; 4) if you're struggling, try King's Row because it has narrow streets which should make it easier. Side note : try to stay on a charter that you hit for at least half a second at least for game to register it(you heal/hurt someone)