How to get the achievement step on her? And Not her achievement?
"Step On Her" requires you to be shrunk in the bedroom along with both Paige and Cass.
"Not Her" is a bit trickier; it requires a specific set of responses during the Miss Collins conversation at the very start, and you only see the results at the very end (i.e. when you see her again).
Hope these hints help, but if you need more let me know!
Okay, spoilers ahead for anyone who doesn't want to see!
For 'Step On Her', you need to stay in the living room and get Bayley to leave, then call Bayley back when you get suspicious about Paige. Then you need to go to the bedroom before things escalate, and from there resist Bayley so she drops the other two, which makes you shrink. Once you're shrunk, hide under the bed, then you need to look for the others and you'll see what happens for the achievement.
For 'Not Her', in brief, when Miss Collins asks if you have any questions at the start you need to choose the negative options, and say that you don't want to be there, pushing her until she gets annoyed and just continues into the apartment. Then you need to follow the paths that bring you to being captured by Paige so she tries to carry you out, only to get into a fight with Bayley. If you escape during that fight and get Miss Collins' attention, she will be upset from the earlier conversation and can be provoked to kill you...