I’ve had it twice, once on Gridlock and the other in Reclaimed. Both in Wave 8 and with Carriers.
The one in Gridlock didn’t pop up until the very end. Our base was upstairs and the team had already taken care of the Carrier and the rest. I walked downstairs to repair a tab and wondered where the last enemy was. I asked: “So where is the last one” but before I could finish the sentence the Wakaatu emerged almost next to me. So the theory that you need to be in the open for it to spawn might have some merit in it.
We had a Marksman in the team that used Icy Precision, which is ridiculously overpowered against it. Freezing damage seems to be destructive for it, unfortunately. I love having it as a boss but I’d wish it to be more resistant against freezing damage to put up more of a fight. Just a matter of time before most people use it.