I know exactly how you feel, I gave up on faction wars after being unable to defeat the first boss, came back a couple of months later with a few 6* champs and he fell down. I'm not totally sure of the mechanics but it always seemed to me that some of his damage is spread amongst your champs so you can't go in with one 6* and expect to win. a couple of level 50's and some 40's should be able to take him down. 

There are different bosses on stage 7 depending on the faction - and obviously the faction also determines which champions you could consider using. So you'll definitely need to specify which faction you want advice for. 

But in general, yes, you're likely to need at least 1 level 60 and a few level 50s, with decent gear. And you may need some specific buffs or debuffs. Speed always helps as well - if you're faster than the boss and can buff your team or debuff him before he gets a turn, that makes a big difference.