As a story progresses sometimes it is helpful to have a bit of help. A Linguist to translate the runes on the fly, a porter or mule to carry all that loot, or a Bard to detail your adventures and inspire courage. Other times they are forced on you. The prince you just saved, the mayors accountant making sure the town gets what it paid for, the little old lady you are helping cross the River Styx.
So to the advice- What can be done to keep henchmen and the like alive and doing what they need to do? As you go up in level and they stay the same what can be done? I specifically am wondering how to keep a Orsheval and Snapdragon Leshy alive in level 10+ Hell's Vengeance and crew in Skulls and Shackles at all levels?
the most obvious way is to not put them in danger, AKA, don't take them with you into the dangerous part, bring them in once you've defeated all the enemies and sprung all the traps. if you absolutely have to have them with you, have them hang back, hide, take cover, etc, very few intelligent enemies will go after someone defenseless and leave an armed and dangerous opponent at their back. but the easiest way is to never put them in danger.
In my first game, one of the other players was a gunslinger who had hired an NPC to be his assistant. He paid with both gold and training. The boy would load the guns and hand them to the gunslinger as needed. He also fired the guns at an opponent if needed.
This is a common problem with the Leadership feat. Even if your cohort is guarding the wagon/mounts outside the dungeon, nasty DM’s will attack them with random raiders or even an intelligent dungeon BBEG might already know the party came inside and has sent a small force to take out the wagon/mounts/cohorts.
At higher levels, contingency scrolls can be really handy. “If anyone approaches within 100 ft of the wagons, or attacks cohorts, or if cohort yells help, or etc,” you can teleport back and help them.
Also, Invisibility works on creatures or objects touched. Wands of Invis can come in really handy, and they’re only 4,500g for 50 charges