A middle ground would be:
"When they die, RESPAWN in d4 rounds. During this time, your GHOST is exposed. Whenever it is hit, roll for GHOST USAGE. On a 1, its current Usage Die steps down one level.
…"
That way the group has reasons to go and cover the fallen ally, and reducing it to just 1 abates the frequency of having to roll when hit.
More or less what I was trying to get at, but much more succinct.
However, I’d stick with the original usage die mechanic where the die step is reduced on a 1-2, because the chance of stepping down the die progresses nicely: 1 in 10 (d20), 1 in 6 (d12), etc., until the d4 and it’s essentially a coin flip at that point. But to each their own, that’s what’s great. If a 1-2 is too chancy, go with 1, it’s your table. 
And yeah, totally, you’d only have to roll that if the Ghost is hit (which again I think should be HARD). At my table I use advantage/disadvantage as well, so I’d also grant disadvantage to the attacker - Ghosts are small and nimble.
Yeah, I see your point in keeping with TBH Usage mechanics as is. I was just offering a middle ground between very hard and too easy to hit.
And yes, HARD to hit definitely! Just not nat 20 because then probabilities are you’d never roll. We want tabletop adventures to have more at stake than the generic videogame counterpart, where your character can never die!
Wild thought that just occured to me: Though players can’t do this in the videogame, the Fighter battles (i.e. the ones Amanda Holloway takes part in) could also be dope in the tabletop.
Dudes.
Mastery from MAGIC, whaddayathink?
Any somewhat-obvious applications to this thread?
I can only think of Void/Solar/Arc energy Mastery playing a role in some way or another.