Casting then hiding

[Stalker0]

Sounds reasonable. Presumably the Spot check would be opposed by the sniper's Hide check, with a -20 penalty?

That's what I would go.

 

[Black Knight Irios]

I wonder how your sniper will ever score a hit at this distance but I am quite unsure about the fact wether the shot target has to make the spot check or not.
When you attack you get out of your hiding place...so you can be seen...but I would say the sniper should get something behind which he can hide something granting 100% cover/concealment.
Do it this way:
Surprise Round:
You are hidden, make your partial action on the surprised PCs then drop behind whatever you are hiding, its a free action, and you are perfectly hidden again and you don't have to worry about the -20 penalty.
1st Round:
get up, MEA
then shoot probably only one attack, you are no longer hidden but you can drop to the ground again as a free action
following rounds:
do 1st round stuff as long as the PCs are far away and retreat when they are closing in, search for a new hiding place and continue the ambush
Seems to be fun...maybe I should use in one of my adventures.