Question : Number of enemies hit by multi-target skill +1. # This inner ability work?

Commerci voyage.

Some of the waves spawn 20 or 25 mobs at once, and there is a counter of monsters killed, so you can see if you kill 15 or 16 of them with one cast (obviously assuming your 15-hitting skill can 1-hit them).

Another possibility is Dimension Invasion, the gargoyle phase. Each wave spawns 20 of them. If you use your ultimate it will only show "10+" for multi-kill, but then when you kill the remaining monsters with your regular mobbing move, you should see either 4 or 5 for the multi-kill.
Again, assuming your ultimate 1-hits them.

Ah, okay. I can test that once the game's back up, as I have the +1 target power from Monster life and a 15-mob ultimate.