I've heard some disagreements on the subject.
Personally it seems like they have a high random chance of doing so after taking enough enough damage, and is instant, even if the mender is getting knocked down.
Other people seem to think that if you attack quickly enough you can kill him before he gets his barrier up.
Some clarification would be nice.
I think it's when their health hits a set fraction. For example, say they hit 10% health remaining. They would raise the barrier then. I don't know when the barrier gets raised, but that's my take on it
Their shields goes up when their health reaches a certain point. However, if the mender is taking continuous damage (blitz needle charge), then they will not shield until the damage ceases, or if they die first.
Maybe when they take damage some amount of times?
I've tried acheron charge with max damage against them in a party of 4 (more specifically in an arena) and they seem to always get their barrier up while riding the explosions. Other times, I think I've seen them putting up their barriers while taking damage from fire. I've been able to able to kill the menders without them putting up their barrier just by slashing away with acheron or using the final flourish charge in a party of 4. The regular final flourish attacks seem to work, but I think I've seen them still raise their barriers when I tried that sometimes.
EDIT: or maybe it really is when their health gets critical like the others said (how did I not think about that?). It doesn't take much to kill the menders after breaking their barrier. Perhaps the stuff I mentioned that gets their barrier up puts them to critical health, and then they become protective. I'm guessing 2 hits from FF charge doesn't put them at critical, but they can't take all 3 hits. I should probably do some careful observation if I see them again...
SK's AI is pretty straightforward.
The menders raise the shield after their health is blow a certain percentage.
However mobs can only act if they are not: 1. knocked down 2. taking damage 3. doing something else 4. dead
If I can do a full Acheron combo on a T3 Gremlin mender they never get their barrier up. Weaker ones will die with a single charge shot without getting their barrier up.
so, if I can keep constant damage coming, I never have to worry about the barrier.
Or charge your Acheron twice and kill them either way, since their barrier isnt that strong.