There are several issues with the whole onslaught villain system, namely the requirements that makes it almost unrealistic to complete in a single day. I often find myself going on for three days trying to get the tokens and targets required to complete it, especially with the fact that turrets can snipe the villain and end their transformation early. The defenders that you need to kill to acquire the '3 kills' to complete the mission more often than not kill you before you can get to the third token. Why the defenders were made this strong is beyond me. If two of those defenders can kill Grond by themselves, why the hell is Grond such a legend? I call shenanigans.
I feel the system needs to be completely reworked to be reasonable, not so buggy, and actually fun to do instead of feeling like a chore.