BINARY DOMAIN

You want to have Charlie in your party from the moment he becomes available until he leaves. Make sure to buy the trust booster nanos from the guy in Shibuya in chapter 2-3. You can use the near-infinite Simian fight at the end of chapter 2-5 to help boost his trust as well. If you want to do this, make sure to NOT have him in your party when you enter the sewers after destroying the Iron Raptor.

You will have a few dialog choices where you can boost his trust in you. Just be agreeable with him. However, this is in not sufficient to get his trust maxed. You'll need to impress him and your comrades in combat.

The easiest way to boost your group's trust is with chain kills, headshots, and one-shot kills. The RAOW-S7 or Yamato-51 shotguns make it very easy to get all three. Whenever you can get the shotgun, grab it and keep it as long as you can. You should only ever switch it out for the sniper rifle or the RPG. Once you're done with it, go and swap back to the shotgun.

When you come across a group of enemies at close-to-medium range, switch to it and charge up to them when you have an opening. Aim just below the head, and fire at point-blank. This is usually enough to get both a headshot and one-shot kill on anything other than the Golems (black shotgun-users) and security shield-using Jarheads. Your team will quickly gush over your rapid kill streak. Make sure you have medkits with you when doing this, as you will get downed a few times. You don't want to rely on your team to rush out into a firefight to help you.

Keep in mind you will likely not be able to get everyone's trust maxed in one playthrough. I got Big Bo and Rachel maxed in the first on Survivor, and Faye, Cain, Shindo, and Charlie (in that order) in the second on No Mercy.