You misread.. Sentinel fire conjoined increases exponentially. Under the surface of onyx thousands of them firing in different directions destroyed the entire planet. Which in includes crust mantle core etc. which is far thicker than you 1 km Death Star armor. All covenant vessels have a primitive form of forunner energy shielding.. And primitive compared to the actual shields of the forunners they are quite strong. Standard shielding on a CCS class battle cruiser is sufficient to stop 3-4 plasma torpedoes then the nanolomite armor they use could tank two plasma torpedoes.. Ok back to the actual fight.. I'd say that 5000 sentinels conjoined would reduce the Death Star into a slag heap. Their firepower and defense skyrocket exponentially.. I'd say their combined fire would be on par with a gamma ray burst.. Death Star is dead. Accept it..