It can be tough...  I tend to rotate the map to help get a better view (or see around a Rook).  I also hover over a target with my mouse and what is under it, I believe glows in a red outline... so sometimes is about knowing the silhouettes of the demigods.

Holding Ctrl also means you can ONLY select DGs for your powers. Means you might hit the wrong DG sure but it won't be wasted on a creep at least. Beyond that, some rotating of the screen etc but I find that most of the time it's not a problem.

{AS got there before me}

Remember that your enemy also has to target in that same mass swarm so it kind of comes down to using Ctrl clicks, space bar and nimble mouse usage

Rotates the camera. Hold spacebar down and press any of the wasd keys.

Or move your mouse.

Or if you are like me with camera movement using wasd, Q and E will rotate the camera.  I did not know about the ctrl button though; thanks for the advice on that one!

You sir, have just improved my game!  Karma!   Thank you sir!