Scheme?

So, you went in that room that has a little tunnel thingy under the floor where the brutes come in? Cuz if you're in that room, did you take out EVERY brute? One time a brute fell into the hole and I had to just revert to checkpoint. If you take all of 'em out, then the door on the right should open. Does this help?

When you get enter the room, kill all the brutes and be sure to look down in the area under the floor to make sure there isn't anymore brutes there. Once you defeat that group you should quickly get back up out of the area under the floor. Another group of brutes will enter the room. If you don't know where they come from, they come from the far, right door (stand in the doorway you entered and face the room, the door is to the far right corner). Kill them and you should be able to enter through that door they came from. Make sure to kill all brutes!

Good Luck!