For laid back sniping (as in, climbing up to some rooftop and picking off an unsuspecting mob at your leisure with critical headshots), try a slag Transfusion (if the grenades do the killing, you’re not sniping so much). Longbow trigger with the lowest fuse time is best here.
Toss the grenade near the enemy you want hit; not so close that the main grenade’s detonation hits them (or they’ll become aggro), and not so far that the child grenades can’t find it. As soon as you toss the grenade, go into Decepti0n, making sure to toss your hologram somewhere the enemies can’t see it (usually behind you), and aim down your sights (get the Kill C0nfirmed stacks going).
When the child grenades hit the enemy while you’re in Decepti0n, the enemy won’t become aggro. Wait till the last second for the higher Decepti0n damage and pop your now slagged enemy. If you kill it in one shot, the other enemies won’t become aggro; enjoy the reload bonus from Killer (to keep 0ne Sh0t, 0ne Kill on deck), maybe a little cooldown bonus (from Grim), and go after another.
You only get to do this as many times as you have grenades, but once you max out your grenade slots, you can drop much of a mob this way. It’s a fun challenge to see how many you can clear before you slip up and they see you. Not that they won’t “forget” you if you lurk in the distance long enough, but still.
There are tons of places to do this, but I think the Beatdown is my favorite - lots of very high rooftops there.