well it took me a hell of a lot of tries, but i beat him by myself on insane with just a longshot and a lancer. the trick to snipe him as much as you can (get as many perfect active reloads as you can), and when he gets to the concrete block which is your cover and starts to move around it, you go the other way. dont go right to the other end, stay in the light about 3/4 of the way along. RAAM should then start to follow you along the barrier. when hes about to come around that side move to the other side again, blindfiring with the lancer the hole time; again, aim for perfect active reloads. If you get the timing and placing right, RAAM should never get around this barrier, so eventualy you'll kill him.