Like a "duel" kind of mode vs specific people?
I think they might have not implemented it because they didn't want people getting free kills for ranking points or coins.
But I don't think that reason is very well founded anymore, since people get insane ranking points and coins from losing and being 8th place all the time anyway hahaha.
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When I play team games, I won't try to kill my teammates (on purpose, unless we gain a 10+point lead and the other team isn't entertaining anymore), but I won't trust them to not kill me - so treat everyone with equal caution.
Same thing in FFA.
There are a bunch of 'skill' moments that show up in 4+ player matches - for example when you're in a quarter of the map trying to kill the leader - both of you are fighting each other while dodging other peoples' bombs. Sometimes I run into these 1st-3rd place players multiple games in a row and we compete in both trying to get more kills and trying to kill each other. I've befriended some legitimate top 10 players and sometimes people are like "oh god it's you again" when I enter rooms because of moments like that XD
I always try to single out who the next possible threat to taking the lead is and try to suppress them. There's a lot of reading and countermovement that goes on when you're trying to trap them in that kind of setting. In FFA, I'm usually in a room with 1-2 other skilled players and we end up killing everyone else while trying to kill each other.
Close range trap-assassination is where it's at :P
http://www.youtube.com/watch?v=wPkHi5xjLLY