The description said it will refresh the duration, but it seems like it also refresh the stat, so every time I used it, it will change based on the current one, instead of the highest one, I am not sure if that's intended or not.
The troll encounter I mentioned is in Vraang, the second mission "ask around", I can't even kill the first one so I was trying different method but the result didn't change too much, the mastery is mostly complete at my current state so there's not much to improve here (I'm stuck at tier II damage spell mastery, since I don't really have a way to reach higher damage even when facing the elemental guards).
The effect for the artifact is quite minimum, even going extreme isn't going to change anything, not to mention the randomization...
the level cap is still 20, so I don't have any other way to get more stats or perks, currently I'm leveling attunements to see if that helps, but it's very time consuming which is why I'm taking a break, I will trying it again tomorrow.