Go to a froglok captive at ( -694, -137, 688 ) /waypoint -694, -137, 688 and use the elixir on him from your inventory. He will attack, don't kill him and he will eventually succumb to the sleeping potion. Right click and "use" him, this puts him in your inventory as a sedated froglok.
The froglok captive social aggros the Rilissian slaver nearby if he's pathing near when you use the sedative. This is likely a bug, but be careful anyway.
If you happen to kill the froglok, simply delete the quest and get it again. You will get the Sedative once you accept the quest since you have all items needed.