I have just beaten Insane II challenge in Brawl Stars Oni Hunt! It was difficult, but I didn't give up! I will say: Wasabi power on Piper is simply incredible!

And about these attacks:

They were just ridiculously easy! The game designers didn't know that I'd been playing Undertale for most of my life and even played a little bit of Cuphead, but even a mountain could dodge them! Sometimes I just stood there and did nothing and the attacks just didn't touch me and passed me by! I just don't believe they call it "difficult", you just have to stay away from Oni Kenji and everything will be okay.

So, to sum it up: it's not a difficult challenge at all if you know how to play. And a little tip that will help you in all games: play with your hands, not your feet.