I think AC2 had a nice mix of this. Also, Gears Of War Horde mode and the Left 4 Dead series give you mass combat but since the enemies come from a variety of powers and skills, you can have duel moments along with mass combat.
In Assasin's Creed 2, you can fight a medium sized mass in combat, not 40k scale, but more then an FPS where you gun down three guys guarding one room, but personalize it with counters, weapon theft, dodging, and action cinematics. The assassinations lend themselves to mini duels with your target(Although AC1 was much more prone to this, even if mass combat did suck dick in AC1) if you don't feel like taking the super easy way of actually being a stealthy assassin.
Honestly, I'd prefer duels as it usually involves more cinematic moments, grace, and adrenaline, it's easier to get immersed. Whereas too much carnage would be like Too Human, sure your moves are cool, but it's the same mindless masses of dumbasses to kill until you hit the boss fight and your moves always have the expected result.
