Lore-wise, I don't recall anything explicitly said that mentioned the Sentient foot soldiers being particularly hard to kill. We know the Orokin initially struggled against them, because they were so high tech, and the Sentients basically hacked them. Can't shoot your plasma gun if the software just refuses to fire. Even the bomb the 157 carried clearly was digital in some way and not mechanical, allowing Erra to just deactivate it.
But other than that? Ballistics have always been good against them, and the Old War feasibly could have been won by the Orokin because the Sentients were running out of manpower (they couldn't reproduce after they made the jump to our system). Warframes were so effective because aside from being one man armies, they were also disposable one man armies. Destroy a warframe? A new one is crafted and ready to go in an instant, and it's operator (dreamer) is not at all harmed.
Kill a sentient drone? That's a fragment of the main body that isn't coming back, at least, not until they get some Kuva into their system.