If you have done the Horror of the Deep quest from Skyzhay's blog

They can be obtained by cheap OSRS GP traveling to Zeah. You can make this happen through a phone call to Veos who is in Port Sarim. After that just run down the beach to the south until you find Sand Crabs. They are very easy due to their terrible defence and high health pool. Be sure to have the best quality armour that you can for your level, and keep an inventory full of food since they can be hit from time-to-time.

Once you've completed Bone Voyage quest you can relocate on to Fossil Island to fight Ammonite Crabs. Those are better than their Sand cousins due to their additional 40hp. Thanks to that you can take a few more turns fighting them , compared to previous creatures. If you don't have prerequisites of Bone Voyage or just don't want to take on the task, you can kill Sand, Rock or Moss Crabs as long you'd like. They will give decent experiences at all levels.

If you have done the Horror of the Deep quest you've also gained access to Dagannoth cave. You can set up a gun there and begin destroying Dagannoths using Rune Crossbow and Broad Bolts. Bring along Prayer Potions, Ranger Potions and the best gear you can put on and runes for Bones to peaches spell.

Because these beasts are both melee and rs3 gold ranged, you have to change your prayers according. They could cause some damage however you are able to regain your health through the fruit you've prepared. This is a great however expensive option, and should you not be able to afford it, just go back to killing crabs.