Grenwalls are monsters that can be caught using the Hunting skill.
They give 1.1k hunting exp and require 77 hunter to catch.
Other requirements are that you have access to Isafdar, the elf lands. This means you must of done The Underground Pass.
You can also use a superior locator to get there.
As you can see, grenwalls are trapped by using box traps with pawyas as the REQUIRED bait.
The best way to hunt grenwalls is to have 3 of the box traps hunting grenwalls and the other 2 to hunt pawyas.
Pawyas are not to be confused with papayas. Pawyas are required bait for grenwalls. Papayas are required bait to catch pawyas.
Because of this, 69 Summoning is STRONGLY recommended as you can cast the fruit bat scroll to receive more papayas.
When you loot a grenwall you will get 18-21 Grenwalls Spikes and receive a valuable herb such as a ranarr.
Profits:
You will get around 1k grenwall spikes per hour which are worth 1,135 gp each. You will also receive around 100k of herbs each hour.
The reason grenwall spikes are so valuable is because they are an ingredient in extreme ranged potions.