Here's how I did this:
First, just play through the game. You might get it on the way. If not, don't worry. Having a fully upgraded "pull" ability, beam weapon, and cloak will help greatly with this.
Once you have beaten the game and have some upgraded abilities and weapons, use chapter select and pick "Access Corridor", Checkpoint #6. There are easily enough enemies on this checkpoint to get a style rating of 30. Just use biotic abilities in combination with your weapons whenever possible and constantly switch weapons so that your "style" counter keeps going up every time you get a kill.
Use "pull" whenever possible (if maxed out you can pull three weaker enemies at once) and rack up kill combos by shooting them all with the beam weapon without exiting out of the game's aim system. You get extra style points for kills in combination with biotic powers. Once that's done, switch to the sniper rifle and take out a weaker enemy to keep your combo counter going, and then use the beam weapon for larger combos when you have the chance.
This level will have a combination of cerberus soldiers and geth, along with one Ogre mech and a bunch of Geth Juggernauts. These can all easily be dealt with using a maxed out beam weapon. Use cover well, and use cloak and run away if you get into trouble. TAKE YOUR TIME. Time is NOT a factor for the game's style rating, so just stay alive and kill the enemies in varying ways and you should easily get a style rating of 30 on this checkpoint.
This achievement will pop instantly once you hit "30", so once that happens just finish the level off or exit out, and you're good to go!
Let me know if you have any suggestions or additions to this and I'll be sure to add them.