EDIT: Check out JOHNNYx76 solution for help with the Husks before the Praetorian fight that I show in my video. There are some nice tips in his solution so please take the time to thumb up his solution if it helped you out!
You get this achievement when you finish the "Go to Horizon" mission. At the end of this mission is a boss: Praetorian. I've posted a YouTube video I made below to show the actual fight on Insanity (I even said PS3 Trophy lol...).
The basics of this fight are pretty simple:
1. Praetorian is usually Armor.
2. Praetorian has a Barrier regeneration move that replenishes Barrier.
3. Don't use cover in the traditional sense - stay off the cover and shoot through openings and over top the cover to keep from getting hit.
4. Run (press and hold A) as less as possible. The reason being is that Shepard likes to drop down into cover every time you're within a few feet of it.
That's pretty much it. If you have ammo for the Collector Particle Beam (gathered earlier in the level), it is very effective against Praetorian whether he has Barrier up or not.
If you don't have ammo for it, try to stick to the Assault Rifle and Sniper Rifle as these are the most effective vs. armor.
Use the incendiary Ammo tech. I'm not sure if this boss resists it (it doesn't according to the radial menu), but equip it just in case as it's effective against armor.
That's about it. Good luck!
P.S. Hopefully the quality will improve on the video below. I'm getting a message from YouTube saying it's still being processed.