Okay, just to give the benefit of the doubt, let's assume the orogs surround the fighter - so this is a worst case scenario where the fighter has done zero tactics. Also, we'll make the fighter a champion, the most vanilla of the sub-classes. So the AC will be 23, since they would obviously take defense to go with dueling.
Round 1: Fighter action surges and kills two in the first round, which is fine because only eight could have attacked them anyway. Those 8 do 16 attacks, hitting 20% of the time against AC 23, so that's 32 damage. Fighter is at 152 HP.
Round 2: Fighter action surges again to kill two more and then pops second wind, healing 26, so is at 178 HP. Orogs do 24. Fighter is at 154 HP.
Round 3: Fighter kills one more leaving 5. Orogs do 20. 134.
Round 4: Four Orogs, left, to do 16 damage. Fighter is at 118.
Round 3: Three Orogs, 12 damage. Fighter 106 HP.
Round 4: Two Orogs, who really should be running away. 8 Damage. Fighter 98 HP.
Round 5: Last Orog. Fighter 94 HP.
Round 6: no more Orogs. They managed to get the fighter down to about 50% health. Honestly, though if they ever managed to get the fighter below half health and they would have been healing 8 HP/round, even with their pitiful constitution. And if the fighter was able to get any kind of cover, such as fighting from a doorway, they could basically tank infinite orogs.
So the only way those orogs are taking down a level 20 champion is basically if there is an army of them and the fighter is completely surrounded with no where to go. Even then it will take them awhile and a lot of dead Orogs.
Which seems about right, frankly.