Don't have a friend to play with? No problem!
You can actually do this quite simply alone with two controllers and two profiles, and still unlock the "Friends with Benefits" achievement at the same time. This method also works for the "Two for the Road" achievement.
Simply set up a local split-screen game with your main profile set on Insane and the other one Casual difficulty. Once the game starts focus on playing with your secondary profile, Dom, which is set on Casual. You'll need to make sure that your main profile, Marcus, is in a safe spot throughout the game.
This is easier said then done though, but not as hard as it might sound. As a first note remember that several times during the game Marcus will "teleport" forward as Dom progresses, so you'll need to reposition him frequently. Once Marcus teleports, pick up your primary controller and hide him behind some cover, preferably as far away from enemies as you can.
This is easy throughout most of the game, but impossible during encounters such as when fighting Skorge or as Jack sets up the beacon in Act IV. It's been a while since i did this, but I'll try and give you a few pointers as far as my memory serves.
Act I (Hotel and Rooftops) - As you split up, press RT to do Rooftops with Dom. You'll need to kill a few enemies with Marcus here as well, but most of them can be reached from the other side as Dom has a Mortar to blast them with. Not that hard really.
Act IV (The Beacon) - Where you need to protect Jack as he sets up the beacon, simply walk Marcus behind the large pillar to the northeast of the circle in which the beacon is to be activated. He'll be safe there during the entire fight, including when the Reavers attack.
Act IV (Skorge) - Prepare for a few deaths. What I did was to put Marcus in the bottom left (southwest) corner of the room. It is however likely that Skorge will throw Ink Grenades at him, so you'll probably have to pickup your other controller and move him over to corner on the opposite side of the room. I did it in 3 tries, so its doable.
This method works perfectly and saves you from having to find a partner. If i can do it, you can do it. However it does require some patience, though I still think its faster than with two people playing on Insane.
If you've got questions, feel free to contact me on LIVE.