When Trip is attacked by common mechs she will use an Electro-Magnetic Pulse to stun them. During this time Monkey can then attack the mechs unimpeded. The EMP takes a few minutes to recharge, so Trip is vulnerable then. Don't let them wake and get her!
After saving her for the 3rd time it will pop. One time in chapter 2 this is scripted to happen and that counts toward the achievement. Another is in chapter 10.