I could see all of them, and the blue orb that shows that I hit them and COULD see them showed. It doesn't show the blue bubble hitting the mob if i couldn't see them.
Also, there are many times I can Mongo through solid objects like walls or raised pieces of land that would otherwise say i cannot see the target if i were using a ranged weapon.
I did that, a LOT. I'd stand still, cast Mongo Slam (or whichever that 2nd one is that heals for 100hp a tick), wait for all the mobs to be hit by the blue bubble, move a little, cast again. Sometimes it helped, but most of the time when I had a mob ignoring mongo in the Hive it didn't matter what i did or whom moved.
What's up with that
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ps: Your name, did/do you live in Aiken, SC :P? I used too ^-^