Krone’s sentinel is Dr. Aiden Krone‘s base of operations. It looks like a robotic spider whose length and height are enormous. Krone must be in there the whole time.
The Sentinel
The sentinel's weaponry are three plasma cannons on the top and one huge plasma cannon in the "belly".
The Beta Suiter 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 Beta Suiter 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 Beta Suiter.
Each cannon on the top 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.