There are different game types that I found provide more experience then others two them being submission and guardian. Since there is no limit on respawns in these games you can rack up multiple kills. With a team of friends who are set on just getting points (been there) we used to just stop the enemy from from delivering the hostage in submission and just get kills. With guardian we would just guard our leader and not worry about killing theirs
However your absolute best bet for getting to 100 is to take advantage of the bonus xp weekends. I made it to 100 just before the gow3 beta so I don't know if they are still doing this but once a month or so they would have weekends where they gave bonus xp. It started with 2x then went to 3x then 6x, I remember the last one I played was 20x xp. You will get the equivalent of playing for months in one hardcore weekend of gaming during 20 times xp.
In fact I just checked and the last one was July 4th for 21X xp source. So keep your eyes open for August. Based on past events I can pretty much guarantee Labor day weekend this year will be bonus XP. Thus far every holiday since they started bonus xp has had bonus xp weekends. Also since it is so close to the GOW3 launch it's HIGHLY likely Labor day weekend will have bonus xp (if not the for the whole week).