Done here as well, mashed it out yesterday.
Had to swap RR in for the grind (could do the boss without her in the first team as well but this sped up things rather nicely, but oh boy. Mace Loli and RR combo... if luck procced, managed to cut through the 2 first stages and half dead nuke the 3rd one.
As long as you have enough single target nukers and a Negate girl to buy them one turn the boss becomes a non issue.

Soooo...what was the final reward the 300K one ?
I thought the 250K one was a 5* ticket. Instead of a girl though I got bloody RG shards, and just 4 of them. Wow why even make that into a ticket, just give it to the player directly lol.

Good dual event. With the Nae chan and Mizuworm rewards combined I managed to pump up some of the girls that were stuck there through the first Ampy stage, and into the Super Ampy tier.