This might be the worse part of the update.
A 1/4 chance to fight the boss that you need and another 1/4 to get each of the items you want.
You can mitigate the pain with multiple atls and using Bubble Gun and maybe HE Bubble Gun.
I have 2 different suggestions for you consideration:
Get 1 of each of the 26% items as rewards for completing. (So you can get up to 2 times the items per run)
Make the 26% items fix rate of something like 70% that can not be increase with Bubble Gun.