HP/MP based state affliction

Hey there, seasoned noob here, not delved particularly far into scripts yet but has a pretty good handle on the functions of vanilla VXace.

The question I'm posing is this: I have a battle system that uses the same amount from both TP and MP gauges to activate skills. In this sense the MP bar is stamina. What I want to do is create a perpetual state that sticks with the characters when the MP (or in this case stamina) bar goes below a certain percent to show fatigue (say once the 'stamina' has fallen below 10% the character loses AGL and attack success is halved until 'stamina' has been restored).

Unfortunately I cannot for the life of me figure out a way to do it. Is there an option I've missed? If that's not the case, whereabouts am I even going to start looking in the script?

If this has already been answered, I apologise. I have no clue where to look.

Thanks in advance guys.