Gloom Stalker, has Umbral shroud (Class invsibility once per short rest). You get it at level 3. Then you keep playing gloom stalker until Level 5. Then you switch over to Assassin at level 5, 7, 8. Umbral shroud gives you at will invisibility. (You just need to find a lightly or heavily obscured spot). Invisbility allows you to get by any enemies, and always 100% attack with advantage and it will trigger a surprise round (The only enemy that can see you is a scrying eye so just becareful with those, and get rid of them from outside line of sight). Duegar get at will invisbility at level 5 (So you can play assassin with them).
These are the only creatures that can see invisibility.
https://bg3.wiki/wiki/See_Invisibility_(Condition)
(Steel Watchers can but they need to use a a bonus action in combat to trigger it and start tracking invisible targets)
