Stop thinking about multiclassing and sort of. I give you the most optimize and easiest way to play Rougue. Play Durge + pick Dark Elf + straight assassin 12 level with him.
Assassin only worth to take if you have the most OP item in this game: Deathstalker Mantle. What this item do is to make you invisible right after you kill someone, no check, even in clear area. This means as long as you kill someone, you can always get out of combat to 'surpirse' the enemies again.
So, this is what happens in Tactician mode, Goblin Camp. We have lvl4 Durge with it, 4-11 normal Crossbow, and a feat to increase Dex to 18. That's it. If you do not get to lvl4, just kill the random lvl2 Goblins nearby.
Our enemy is the miniboss True Soul Gut who has 52HP, 14 AC.
First, we sneak somewhere a bit high, but still sneak and attack him in turn based mode -> 11 - 18 damage.
Suprised attack -> we get to attack again, this time, it will be a crit -> 2d8 + 4 + 4d6 -> 10 - 44 damage. (for whatever reason, it is 4d6 instead of my PB here)
Then, because we have +5 initiative -> we get to attack again for the 3rd time, hooray -> another 11 - 18 damage.
-> 32 - 80 damage. Turn off karmic dice and you can get the average value to kill him. Even if you do not, just change to melee and slash him when he dashes to you.
Yep, you literally have 4 turns to kill him as assassin (not 2 like most people think), and if he is not dead by then, you are doing something wrong.
All these attacks have 88% hit rate and if you climb on the beam, 94%.
Then, you think, now, what happens after you kill him?
Nothing. You are invisible now. Just run straight to a place that does not have any guard, wait a bit, rinse and repeat until you finish the game.
If you feel guilty and need some redemption because you play the game like Assassin Creed, you can multiclass into 5 Ranger to get the Talk With Animals for Scratch to ease your guilt.
A lot of boss combats begin with a forced cutscene, so you get no surprise round. Along with lots of enemy types in act 2 and 3 that see through invisibility and cannot be surprised.
