Where is the best place to level Staff and Grenades?

[ruisen2000]

I really wouldn't level a staff or grenade on a boss. In my opinion, you get a lot less XP that way.
I maxed both my staff and my grenade in the Hallway (with the 4 guys) in tormenta.
If your notoriety is level 48+, and your grenades are in the lv 25 ish, with max on siege and the grenade version of take aim , you can also try bombing the 4 guys in treasure room with Siege.

Also, there's a place with 4 ghosts lined up in a nice, straight line in the mines in rc that can provide you with some nice xp.
Don't look at the xp from an individual enemy, look at the overal XP you would get in 1 hour.