What works with Archer FA?

That isnt an Archer...

It's the exact same thing. I think the hit animation is even the same. A monster in range will just suddenly take damage. It does not work with Inferno, Blizzard, Arrow Rain/Eruption, or Power Knock-back. It can only hit one monster, regardless of whatever skill you're using. If the monster you're attacking dies and FA activates, it can hit a different target if they are in range. FA can activate Mortal Blow, which is, in my opinion, the most useful aspect unless you're a Bow Master. It is otherwise completely unreliable to guarantee a certain Hit-to-Kill that you otherwise would not have without it, especially since most training is done with mob skills, and killing one monster out of six one hit sooner means that you're still using the same number of hits to kill the total six monsters.

tl;dr Final Attack sucks unless you're activating it 3 times per second.