We'll Be Back Achievement in Halo: The Master Chief Collection

In order to achieve the par score on each of the nine missions of Halo: Reach's Campaign, you will want to take two things into account. First, skull modifiers and difficulty setting will greatly impact your score; the more skulls active and the higher the difficulty, the higher your multiplier. Second, you should aim to complete missions as quickly as possible (ideally within par time, as it will give you a x3 multiplier) while still killing as much as possible on the battlefield via headshots and multikills. If you do happen to run over par time, try picking up the pace as the time multiplier will begin to reduce significantly.

Once you beat the Par Score for all ten missions, this achievement will unlock.

NOTE: It is not absolutely required to play missions with skull modifiers active in order to earn their par score. In fact, most of these achievements can and may come naturally during any Legendary playthrough given the difficulty setting. If they don't, though, it's then easiest to attempt them on Normal or Heroic with a few of the "easier" skulls active (Catch, Famine, Fog, Thunderstorm, Tough Luck, etc.).