Hey there. Quesooo. To give you some clarification, your lubs should look like this:
npcidentity entry:
["JT_MONSTER_LAST"] = 3300, <-- This is your current last monster ID. Whatever it's set to, boost it by like 100. Never let the value of the other entry (see below) surpass this value.
["JT_CUSTOM_MOB_NAME] = 3200, <-- Input the new monster's sprite name and ID in the lub here. Example: If the last official(/non-custom) monster ID is 3199, give your new monster an ID of 3200.
jobname entry:
[jobtbl.JT_CUSTOM_MOB_NAME] = "Custom_Mob_Name", <-- Sprite & mob name goes here. Keep in mind, the part between the quotes must be its exact sprite name. It's case sensitive, so if the .spr and .act do not have any capitalized characters, do not include them.
Then of course, the entry in the mob_db.txt should correspond to the above info.
Best of luck.