Well it all depends on what youre best at playing and how many people you go in with. If you can't snipe don't play snipers. If you hate FFA or you can't perform well at it then don't do lonewolves. But if youre going for a legit 50 with a team it can take awhile. I have a legit 50 that took 152 wins with 24 loses in team slayer. But at the same time I have another account that has 61 wins 3 loses and and I've been dropped from 26 to 24 because of the loses. It's tricky to say whats easiest cause if you go for legit 50 expect there to be bumpy areas and setbacks. You'll realize that the true skill system can frustrate the hell out of you.