I run Promethean Code on hard. There are exactly ten taken majors/ultras. It's also useful for things like get 5 kill spree or 5 kill spree with heavy weapons or special weapons (the taken thralls at the start). I actually run this one for a lot of bounties. If you have a charged agonarch that'll do it- but is a bit more effort. I used to run around on the dreadnaught- but it's a pain to find which area is spawning taken next.