L/Cpl Edward Maher was discovered sitting in the Brecon Beacons "like he was just having a rest", an Army medic said.
The medic then spent 45 minutes trying to unsuccessfully revive the soldier.
The coroner heard L/Cpl Maher and two other reservists died during or after a 16-mile march on 13 July, 2013.
Describing how L/Cpl Maher was found near a checkpoint, the medic, referred to in the inquest as 1N, said: "He was sitting in a completely natural position, almost like he was just having a rest.
"He had a half-eaten chocolate bar in one hand and his water bottle in the other."
The team medic then began CPR and requested an immediate "casualty evacuation".
Timings generated from L/Cpl Maher's emergency beacon suggest that 1N found the Hampshire-born reservist at around 4.45pm.
He estimated he battled single-handedly to revive the 31-year-old for around 45 minutes.
He said: "Two medics were despatched from checkpoint four in a Land Rover, however they got 'bogged in' trying to access my location."
An air ambulance which attended L/Cpl Craig Roberts, who also died, arrived at the site where L/Cpl Maher was found at about 5.30pm and he was pronounced dead.
Earlier, the inquest heard that a passer-by was asked to fan down Cpl James Dunsby, who died in hospital from multi-organ failure.
Cpl Dunsby, 31, died at Birmingham's Queen Elizabeth Hospital on 30 July, 2013.
In a statement, a senior paramedic with 23 years of experience said Cpl Dunsby's temperature was the highest he had ever encountered.
A Welsh Ambulance Service worker told the hearing that Cpl Dunsby's temperature was recorded at 40.4C (104.7F) and his pulse rate at 138.
The hearing in Solihull, West Midlands is continuing on Wednesday.
An Army reservist who died amid "heatwave" conditions on an SAS test march was found still clutching a bottle of water, an inquest has heard.