There's 2 of these mastodon, one at the entrance and one past the bonfire with a rat, they're basically in the same place. Can find more mastodon later. But yeah the limited spawns suck for farming items without using ascetics which can be very annoying since they also buff a bunch of other stuff. For example I farmed the statues in the castle right before the mirror shield boss, I did it cause I just wanted to clear the fuckers so I could try the boss in peace, there's one spawning right in front of the fucking fog gate so running back was annoying plus I noticed they dropped slabs. Ended up with 9slabs which is cool but only one weapon(stone twinblades) and one llewelyn armor piece, the chest, which is fucking awesome btw. There's 4 of these mobs and I farmed them with covetous ring+1 and jester hat and yet that's all I got, so kinda shitty if I wanted the whole set or if I didn't get the chest. I used the chest for the rest of the game and it's by far the best one I've found, in terms of protection/weight. It's like chainmail weight but offers much higher protection than everything else in that weight category I've seen, plus it looks cool as fuck, like leather black armor with a cloak and shit, kinda like game of thrones night's watch's armor.
Using an ascetic there I assume also respawns the boss, so it's not viable to do it unless you're already fairly far in the game and can actually kill a NG+ boss np. Granted it's far enough in the game that it should be possible, but still point is you can't ascetic only certain mobs so it's quite annoying. At the same time, with how many bosses have fucking mobs RIGHT ON THE DOOR as opposed to a bit in front of the door like in DS1, I don't mind it too much since running back to the boss sucks if you don't annihilate the mobs.