Egg On Your Face - 15
Defeat a Majini with a rotten egg.
Drop the difficulty to Amatuer and start chapter 1-2. Run through to where you encounter the female Majini, when you've got her attention shoot her with the pistol once and quickly equip the rotten egg and throw.
A Friend In Need - 15
Save partner 10 times when HELP is displayed.
When your partner runs into trouble and is grabbed by an enemy the HELP icon is shown on screen. Run over and press to assist. 10 times and this will unlock.
Lifeguard - 15
Save partner 10 times when DYING is displayed.
Easiest way to do this is go onto a higher difficulty and don't give Sheva any weapons or health items. She'll soon get into trouble and when the DYING icon is displayed on screen rush over and press to resuscitate her.
The Works - 15
Chain the maximum number of combos together in one go.
Who Do You Trust? - 30
Build up a certain level of trust with your partner.
Thanks to GaMeReVX.
Originally Posted by GaMeReVX
You can actually get it quicker than that. I basically loaded her up with healing items, got hit and let her heal/resuscitate me.
The key thing that I did during this was to say "Thanks!" (Press
after receiving an item/being healed from Sheva) after any interactions. No, seriously. I have yet to play single player (been playing co-op) and all I've been doing to get the achievement was thanking her whenever she healed/resuscitate/gave me ammo. It took approximately 10-15 interactions (unverifed exact number, apologies due to trading ammo as well, also took about 15 minutes overall, majority healing interactions with each other) to get it.
(Notes: Ran around Chapter 1-2, getting up to area with the boss battle. Ran the chapter twice to obtain achievement, getting it halfway on second run.)
Hope that helps!