There are some really good modifier storm missions out there, I got 6 + 12 drops of rain from one of them – and they’re not super hard. They do expire though so you have to jump on them. 18 mother-flippin’ drops of rain from a single mission! They tend to be a bit harder because of the weirdo modifiers (the slow on hit modifier is a particular kick in the ass, especially combined with the acid cloud on death one too) but nothing super tough IMO.
I got another legendary hero finally, some electric base dude. Thanks to the deluge of drops of rain (praise RNGesus) I’m leveling him up to use in my alternate slot and I am also levelling up the other legendary soldier I got as part of the founders pack. Legendary heroes are pretty tough to get, so maximize the ones you do have.