The reality is that you'll need to complete the campaign on any difficulty setting to unlock Mercenaries mode in the first place. You'll inevitably get some practice here and get used to the controls, which are the same when you transition to Mercenaries. However, it might be beneficial to experiment with different controller schemes for this mode. Overall, the enemies go down quicker, and you will need a higher focus level than the campaign with regular appearances from stronger foes.