Assassins are more for 1v1 because of their extremely high attack. Bringing a sin to mass PK is asking for trouble because their defense without refines and proper shards is terrible, as soon as you un-stealth you are leaving yourself open to attack and because your defense sucks people can pick you off very quickly if you can't get yourself stealthed fast enough.
As for Seekers, I don't know much about them but I do know that their AoE attacks are probably more suited for mass PK. This doesn't mean they can't make for a decent 1v1 opponent, they are pretty dangerous either way.