So here goes my first bloging attempt. As am probably not only person wondering how often acorns drop for current Cover Task I tried to get this droprate somehow.
Some general info. All fights have their combat drops. They are calculated for each hit and KO of enemy. It seems that each fight has 4% chance for generating drop on hits and 15% chance for drops on KO.
|Fight||Hit chance||KO chance||Drop rate|
So what would be best strategy for Acorn farming? Grab heroes with lot of hits per attack. A good blog post was written by OmarFPG. Equip your hero with Minigun. And hit mission in Season 2. Which ones will do? Considering drop rate you should be aiming for one with most boss fight and high threat missions. Also three-birding seems to be less efficient for Acorns farming.
As for me looking at fight without much calculations I'd go for following missions:
Other missions have less High Threat missions or just one Mini-boss. Not really good at creating statistics so can't really tell if those are moe efficient paths, just my assumptions.