There is no evidence that Russia killed 762 Ukrainian soldiers in one go using a TOS-1A. Neither country has claimed so, nor is it verified.
Recent social media posts claimed that a TOS-1A (rocket launcher) killed around 762 Ukrainians during an attack. A Facebook post reads, "SOURCE CNN: RUSSIA FIRED 320 MISSILES USING TOS-1A IN SINGLE STRIKE 762 UKRAINE SOLDIERS DEAD IN DEADLY STRIKE*," with the claim that CNN reported it. CNN has not reported any such incident since the war started.
TOS-1A is a Soviet rocket launcher that is attached to tanks. According to Reuters, the rockets fired from this weapon are around 15cm long, have a firing range of 400m to 6km, and contain about 24 to 30 rockets. Though Amnesty International and Human Rights Watch banned cluster munitions, it was reported that Russia used TOS-1A to fire at a preschool in northeastern Ukraine while civilians took shelter inside.
In February 2022, CNN reported that it spotted TOS-1A in Belgorod, Russia, near the Ukrainian border. The report also mentioned that Ukraine did not identify the weapon being used in the war. However, CNN did not report about Russia firing 320 missiles using TOS-1A. Whether such an attack occurred or whether it killed over 760 Ukrainian soldiers is unknown.
On May 29, 2022, the BBC reported that Russia confirmed it was using TOS-1A to target Donbas. Russian officials told state media that Russia launched its latest multiple-rocket system against Ukrainian targets in the Kharkiv area.
There are no reports from CNN or other credible news outlets of a TOS-1A killing 782 Ukrainian soldiers. Recent attacks using TOS-1A are yet to be confirmed and verified.