Item description:

killed every monster in both large and small sizes except for Odogaron. There are tons of investigations available where he spawns so you shouldn't run out. The real problem is that all the youtube videos out there never clearly show how big a Large and Small Odo looks like because they're either on a slanted plane making his leg bend look bigger than it really is compared to the player, or they just rush in and kill it without showing any distinct measuring references.

So for large Odo trap it on a flat surface with a shock trap and if the first bend to the outer left leg is above the player's head it's large and you should kill it, for small odo standing next him without him being trapped the first bend in the leg should come to slightly below the chest almost to the groin for the player. If you're still unsure it takes 3-5 minutes to kill him so you can be 100% certain.

If you don't like the weapons and armor provided in the save there should be enough materials to make something else. But the exact setup is what I used to pretty much kill everything in the game, give or take some weapon changes.