The sentinel's weaponry are three plasma cannons on the top and one huge plasma cannon in the "belly".
The player sees the sentinel in the first mission, "Arrival". Next, he sees it in the mission "Road Trip". Sentinel is also the final boss in mission 24, "Consequences". There, the player exterminates all plasma cannons and, after the last cannon's destruction, the whole sentinel explodes and falls. Krone then crawls out of the ruins (obviously the only survivor) and gets killed by the player.
Each cannons needs 6 accurate shots of the surge gun to be destroyed ( the last one needs about 10). Each time a cannon gets destroyed (except the last one), the rooftop you're standing gets "bathed" with rockets, so, be CAREFUL.