If you are not going through the process then it is not really giving the NPC a player class.
If you are just whipping up something then you are doing what I suggested, pulling abilities off the shelf that you want to use and ignoring others. I see no value in checking if a hypothetical PC could have that same combination of abilities.
For the example above - a PC with 5d6 sneak attack, the other Rogue/Assasin abilities mentioned, and Divine Strike .... I could build a 17th level PC (Rogue 9/Cleric 8) that got that, but it would also have 17 hit dice, a +6 Proficiency Bonus and lots of spells I don't need. So why do it? Just make an NPC with 7 hit dice, +3 PB and all those abilities.