I have a relatively hard time spawning SGs as well. Yesterday, I even had to do a Character Story map to clear the daily mission for SGs. And it looks like I'll have to do that again today.
I blame the RNG gods !!!
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It's a small comfort that once you've beaten the four 'normal' event bosses the super boss can be fought as many times a day as you have spare tea sets. I bought another set of five tea sets just in case but it turns out my squads were strong enough that I only had to use two of those five.
All of the squads in my boss fighting group are around 130k power (lowest is 129,190 but reduces attack power for two pests by 70% and a third by 55%; not so hot in these fights with a five round limit but great otherwise). Now that I know I definitely can't beat Auntie with a single tea set I'll just use two at once to guarantee the fight ends in one try with the triple damage output.