You just need 7 exp in one playlist. Because of winnng the two Team Slayer matches I played this morning my exp for that playlist got bumped up from 7 to 8 but I never got the 7 on 7 achievement to unlock. Over lunch I went home and jumped in 4-5 Team Slayer matches and dropped out of them to lower my exp back to 7. The achievement didn't unlock on the first match I was in and dropped with 7 exp but it did unlock on the second one I dropped with 7 exp. It seems to be a bit glitchy but it will work.
I know I've read that there is supposed to be a special 7 on 7 playlist on the 7th of the month that starts everyone out with 0 exp so everyone can have a chance to earn this achievement but I couldn't find it. I hated to drop out on my teamates like that but I wanted to get this achievement out of the way for once. Oh well, at least I won't have to bail out on matches anymore.