I got this achievement once I’d deactivated the pumps in the east and west pump rooms. When you return to the pressure control junction a Titan will rip his way through one of the lift doors. Keep your distance from him so you can encourage him to charge. When he charges make sure you are standing in front of one of the electric gates and as late as possible dodge out of the way. With any luck he will run into the gate and stun himself. If you dodge too early he’ll stop in time and won’t be stunned. Use this chance to attack him but stop after the initial flurry or he’ll get a hit in on you. After this, some generic bad guys will appear. Do your best to repeat this tactic once these guys have been taken out. Rinse and repeat until the Titan is down. Intermittently (probably twice) you will get the chance to ride the Titan. When you do this you should be able to get the Freakshow Rodeo achievement too if you spend some time harming the henchmen atop your crazy, crazy horse. Most important thing to remember is not to use your gadgets at all. Just don’t risk it.
This isn’t necessarily the easiest way to get the achievement, just an alternative in case you miss the earlier time that Wottstar83 explained.
Stand in front of the electric gates when the Titan charges and dodge out the way as late as possible. Hopefully he’ll run past you and get stunned so you can beat him down. Repeat.