NOTE: After hearing some issues when people tried this, you may have to increase your renegade/paragon level AND defeat the boss all in one sitting. This game has some weird glitches and likes to reset statistics when it closes, so if you aren't getting the achievement, try doing your renegade/paragon choices and the final boss battle without closing the game. Let me know if that works so I can update the solution to be more complete!
The other solutions work as well, but I wanted to write one with a bit more detail and to clarify a few things:
When you start the game, make a decision if you want to be a paragon or renegade. Stick with that choice for your entire full play through. I'll explain why later. You will NOT have to do two full play throughs.
As you go through the game, you will be offered choices. The first is at the end of the "Ice Giant" level after you fight a Turian sniper and can't be missed. For the rest of the game, look for branch paths off of the main hallways (Especially in the medical lab level). There will be small offices with Cerberus scientists in them. When you enter these rooms, you will be given a choice between "paragon" or "renegade". Select whichever one you are going for throughout your first play-through and keep selecting it throughout the rest of the play-through.
Your biotic powers will be blue if you are currently a "paragon" in the game's system and they will be red if you are considered a "renegade". These choices will affect which of two slightly different endings you get for the game when you beat the final boss.
I suspect that even if you are "paragon" or "renegade" according to the biotic colors, you need to be strongly in one direction or the other for the achievement to pop. That's why I recommend sticking to one choice on your main play through.
Once you have completed either the paragon or renegade achievement, you DO NOT need to uninstall the game and you DO NOT need to play the whole game again to unlock the other one. Just re-load the medical lab level using chapter select and play through it selecting the opposite option from what you used on your main run. Keep testing out your biotic abilities. You will know you are close to being ready for the second achievement when your biotic abilities shift colors (red if you are now going for renegade or blue if you are now going for paragon).
If this solution doesn't work the first time, play through the medical lab again making the same choices and replay the final boss. When I played through and first made the switch, it didn't unlock right away (even though the color of my scars/powers changed). Try to play through the medical lab choosing the renegade options again. Then play the last boss again. I think even if the color is red, you still need to be "more renegade" if that makes sense. When I did this a second time, I got the achievement.
Once that happens, load up the final boss again and beat it to see the alternate ending and get your other paragon/renegade achievement. If it doesn't pop right away, play through the medical lab again making the same choices to "Strengthen" your paragon/renegade status and then play the final boss again. Eventually (after you do this two or three times to shift your paragon/renegade status far enough in the right direction) the second one will pop and you'll be done with the paragon/renegade achievements for this game!