All that worrying for nothing. Compared to all the battles leading up to it, the Praetorian was a freakin' cake walk. There's a truck near the middle of the battlefield that has a bunch of crates on it. It's just high enough that the Praetorian can't fly over it and it's beam can't hit you if you stick close to it. The Praetorian almost always goes after Shepard, so I just kept circling the truck and getting in shots when I could. When it did it's slam/explode attack, I just backed away for a moment and moved in when it got back up. Rinse, repeat, get the hell off of Horizon.