It is my experience that leveling grenades at kingshead no longer works no matter where you stand. The Navy comes through the fence if they cannot bayonet you at the edge of the fence. The Kingshead technique used to work flawlessly, but now it's a bust more often than not, at least in my experience.
The way I go about it starts in Cuba or Tortuga.
Nade Fly traps at either site to begin with--Tortuga has higher lvl fly traps though, so best start in Cuba to begin with. Once you get to like lvl 5, 6, definitely move to Tortuga. Nade the fly traps in Tortuga Swamp area until around lvl 8 - 10 with nades. Once the enemy turn green--not yellow or red--time to move on. That applies to each place I mention here. When the enemy banner starts being green, find another spot to lvl with higher lvl enemies for better rep and usually better loot.
Next -- Go to where the grenadiers and slashers reside in the Catacombs. My tactic, shoot the slashers (lvls gun pretty quickly too) AND THEN grenade the grenadiers. YES, you take damage because they throw at you too, but if you're quick enough (and anyone should be) you can get three, four, or five grenadiers stacked around you at once (the two in the back room would make four or five grenadiers). Then as you throw, you kill them all. I can usually get arounda combined 250 - 350 rep from killing ALL five at once--Around 75 rep for each enemy. That's pretty good. I use nothing but stinkpots and the dead enemies tend to reward you with stinkpots re-ups as you kill them--from loots bags, chests, skulls of course. I dont put "dots" (skill points) into any thing else for a long while. I open up stink pot with one dot and place ONE dot in long volley. I save the rest of my skill points for WAY later (the nade passives and siege), OR stick a few of them in the explosive slot.
Hint: Putting three or four skill points in the explosive grenades skill can help you kill something fast if you get into trouble--about to die. In other words-- for a moment you can switch from leveling mode to life saving mode with simply going from stinkpot to explosive that has 3, 4 skill points in them. Remember when ready, you can always re-train the weapon and take those points back out of explosive and place them where you want--but for now we are just discussing leveling those pesky nades. Anyways, you can get to around lvl 15- 20 nades in the Catacombs and move up 4, 5, maybe 6 lvls on your gun skills.
After the Catacombs, I move on to Tormenta where the flotsams and spinskulls are. I shoot flotsams and nade spinekulss til around lvl 23 to 25 on my grenades. Spineskulls use FLAME NADES--Be careful. Your rep may slow down here because you WILL see the importance of having those 3, 4 skill spoints in explosives here. But they are part of the progression of leveling nades. Never said it was easy OR fast--lol!!
Once done there, you will move to the Thrall Room. Shoot Thrall Captains and nade Urchinfists. Thrall Captains are NO JOKE. So...... ya better have that gun PRIMED--LvL 25 or higher with steel shot opened up and at least a couple of skill points in it. As always, have plenty of tonics wherever ya go.
One last hint--When ya get to Thrall Room--Ya can also DOLL Thrall Captains to0--attune and run--better have some strength there too though. Again, Thrall Captains are NO JOKE.
Take lots of tonics wherever ya go. Also, it's a leveling task that, most times, is best done alone and on a REALLY quiet server.
Okay, there's my guide. It has worked for me with going on three great pirates now. Hope it helps.