Age of Wonders: Planetfall (PC) - Achievement - Critical Failure
Make enemy units fumble 13 attacks in a single battle
Making units 'fumble' is dependent on their morale. To efficiently obtain this achievement, you need to manually battle with an enemy army that is on as low morale as possible.
To achieve this, I opted to play Huxley Wunsworth - An Oathbound Psynumbra Hero. Psynumbra tech has a few operations which cause negative morale effects. Huxley himself gets the Sinister Chorus ability after levelling up which again contributes towards giving enemies a negative morale modifier in battle. This in combination with the 'Vision of Darkness' operation stacks up to a large enemy morale deficit and both abilities are obtainable early on in a game.
It's best to make sure your using an army that you don't care about as you'll basically just need to stand there and take the hits in the hopes that the enemy units keep fumbling until they hit 13 fumbles, but with Vision of Darkness being used beforehand and Huxley's Sinister Chorus, 13 fumbles shouldn't be challenging.