i thought i'd post this for anyone that wants to level grenades and hasn't otherwise figured this out. it's also based on being at the level to throw these grenade types.
while trying to level stuff recently, i've discovered what seems to be a slightly faster method to level grenades. any little bit faster is better than nothing.
start with full grenades, and stand just past (south? of) the gun and sword shops. target Darkhart so that you can see the red circle (although it might just barely be visible). open with a long volley fireball, maybe 50-75% to reach him and not fall short or throw it over his head - neither helps.
when he runs up to you, hopefully he'll stop a short distance away and start throwing daggers. long volley a siege charge at ~50%. if you take damage, you're too close - back up a step. then alternate between long volley fireball at ~75% and long volley siege charge at ~50%. this seems to avoid the "skill not finished charging" message. if he doesn't switch to sword (and stay put), you might be too far away - move up a step, but not too close, or he'll just switch to sword and be done with it. it's possible to get him to switch back to dagger so you can try again, but it takes a lot of running around.
each time you throw one, immediately switch to the other and hold the mouse button down right away - even before the previous grenade lands. i've found that by switching and trying to keep it "charging", it takes less time than using the same one over and over.
when you've thrown 8 siege charges at the same one, stop throwing those and alternate between long volley fireball at ~75% and long volley grenades (#1) at ~75%. over time, i've found that lets me take out at least 5 of him before i have to buy more grenades. sometimes his loot drops include what you're throwing.
if the other wandering undead get in the way, let them; the extra points add up over time. if they're closer to me than Darkhart, i've been known to intentionally throw the next siege charge right between the two of them. it's not your fault if they're "standing too close when the super sauce takes effect".
as an example, i just went from grenade level 26 to 27 (~5,900 points) in about 2 hours. hope this helps!