If you are struggling to hijack an Atlas for the achievement, it's best done during Grissom Academy, where during the last area when you have control of an Atlas, you can use the Atlas' main weapon to break the canopy of the enemy controlled Atlas that comes to fight you near the end, killing the enemy pilot in the process. You can then just get out of your own Atlas and get into the other Atlas at that moment.
It is possible to lock yourself out of a Liara romance in Mass Effect 3. This happens if you choose the lower bottom responses instead of the upper top ones during optional interactions with her. By optional, I mean interactions that you can avoid, like the time capsule scene and the scenes on the Citadel. You can use all the upper top responses you want with her during non-optional interactions, like during Mars and whenever she comes up to your cabin to inform you of something, the responses during those interactions won't have an effect.
You will know if you have successfully locked yourself out of a Liara romance when in her lock-in chat, instead of the final upper top option being "I want more than just friends", it will instead be "I'd like that".