I've made a video for this achievement, it's an easy and quick way to pop this.
ALERT: To follow this guide you must do the small mech glitch, so if you are against glitches, its not for you!
That said I will leave two videos here, one showing how to small mech and other the complete run to unlock the achievement.
How to small mech:
To do this glitch you must place the mech over a save plataform and jump into it and then time your eject correctly with the screen's flash (timing right is key). It's really important to press eject button just once, so you don't leave the small mech in the sequence. Also I found that pressing the eject button then firing rockets would make it easier and prevent from saving every single time you eject before the right time.
Note: after that you shall not eject, die or quit to menu, these undo the glitch.
Sequence Break achievement and heater core completion in less than 5min:
Hope this help you, feel free to message me or leave a comment!