UPDATE: To get this achievement now, you must (as the description states) hit 7 targets
simultaneously. Although the achievement originally unlocked by just hitting 7 target
during your Coalescence, it appears this has been patched. The possibility for this being corrected was mentioned in the original video solution (about 0:57). It's definitely still doable, but now will require better timing. Make a point to use your ult in hallways and stairwells during team pushes. If anyone has specific maps or spots they recommend, please share in a comment below. Thanks everyone!
Original solution: The description for this achievement is actually a bit misleading. 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.