To unlock 'Fish' you must complete the campaign on 'easy' difficulty.
There are seven campaign missions, all of average length.
The achievements are stackable, so if you complete the game on this difficulty then you will only receive this achievement, but if you go up then the difficulties stack.
I would suggest playing on hard difficulty to unlock all three achievements at once, purely because hard is the equivalent to normal in most games, its really fairly simple.
The only tip I will offer, is I would take a sniper rifle from every box. It's incredibly easy to chain headshots up, leading to mega scores for your assignments.