Taking a look now; nemesis devils can be melee brutes with lots of natural attacks. They also have everything necessary for the scry-buff-teleport tactic - scrying doesn't provide enough info to teleport on its own as of Ultimate Intrigue, but if you know the location otherwise it still tells you when to go, etc. Their other tactics are minionmancy with their devil mark ability to give out SLAs, and maybe throwing blasphemy around but that last depends on nonevil enemies which I think excludes most of your party.
So, 3-4 levels to make a natural attack brute better. With mooks. 2 levels of medium channeling the champion spirit is a fair number of buffs (including +2 damage for all the mooks), or 4 levels of unchained rogue can throw around a bunch of debuffs on anyone it can hit with a sneak attack, or barbarian/bloodrager helps it very directly in melee, perhaps skald to drive its mooks into a frenzy...
I think they're supposed to be heavily themed around their domain though. What domain does this one have?
Knowledge or a fate domain which is all about accepting fate serenely; I guess that makes anything with rage unlikely. Something monkly seems more likely, though flurry of blows is mostly incompatible with a variety of natural attacks - master of many styles loses flurry in order to take a couple of styles at once. Maybe wolf style to take advantage of all that reach and efreeti style/stance to set a few people on fire.
Or if you're willing to go for 3rd party, Dreamscarred Press classes from Path of War multiclass really well - with one level in such a class your devil would count as 9th for level prereqs, and so could start with a 5th level maneuver. With 3 levels they could have a 6th level maneuver. Like Tome of Battle classes if you remember those. Warder or Warlord and the Black Seraph discipline seem like they'd fit pretty well.