If you don't care about trash mobs and only care about bosses, why do you prefer to play an assassin? Assassin 1 grants poisons. End game bosses tend to be immune to poison. Assassin 2 grants assassinate. Tends not to work on end game bosses. Assassin 3 grants random insta-kill. Doesn't work on end game bosses. All 3 ranks of assassin grant bonus sneak attack damage. End game bosses tend to have fortification. Duel wielding assassins get a penalty to to-hit for each hand equal to either -2 to each hand or -4 to each hand, depending on various factors. End game bosses tend to have high ac. Assassins excel at fighting trash mobs and suck at fighting bosses.
The best type of rogue for fighting end game bosses is an acrobat. Half orcs I believe get enhancements that increase the effectiveness of 2 handed fighting so if you prefer to play a half orc rogue, your half orc would be well suited for being an acrobat.