The file set for Unyielding Will comes with conditional triggers and finish conditions based on hull percent. Look at the ability and see how it works to get an idea.
This in a file means that the ship this file is being applied to has to have 75% or less hull left for it to trigger the instant action.
This will allow the buff to terminate at the proper percentage. Between them, firing and ending with respect to a particular percentage is very easy.
You are not out to accomplish an easy task with respect to targeting, however. The condition works only on the caster, you'll need to use HullDamageExceedsAmount to make them fire if you want to do this at all in an easy manner, which is horribly sub optimal as ships have a rather diverse range on total hull.
If you want to do it right, and have these function flawlessly, we're talking about interlocking abilities. A self cycling buff applied to every ship as they come out of the shipyard would work perfectly. The easiest is to just have it apply and finish the various stages you need exactly like BuffUnyieldingWill does, and then limit your repair abilities based on the existence of those buffs as is done for the boarding party related abilities in vanilla Rebellion.
I'd prefer something like this myself, but it wont be going into the mod for performance reasons. This will eat a bit of cpu. The messy one is efficient, but it's very, very messy. 75% damage on a Sovereign is more missing hull points than a lot of ships have. There are many ways to accomplish the desired results, but none of them are both efficient and effective.