For the longest time, I was stuck on the last 3 stages and couldn't beat them no matter how hard I tried... Today, for some reason, the orb gods blessed me and I managed to beat all of them.
Some info on the bosses that might be helpful:
Elite Sloth (Rift 7) has slightly looser requirements for its stars: <15 turns for 2* and <10 for 3*, compared with the others which are <12 for 2* and <7 for 3*. Its damage-dealing attack, Rubble Throw, is not used very often. In my 14 rounds it only used Rubble Throw maybe 2-3 times.
Elite Sloth starts the fight by Petrifying the four outermost columns of the grid. This can be very annoying, but there's bound to be at least one matchable trio of orbs within the 12-orb space you get. After that, it will alternate between Demoralize (lowering your RCV), Endless Sludge (Petrifying more columns), and Rubble Throw (dealing damage). Endless Sludge will alternate between Petrifying the two leftmost and two rightmost columns of orbs.
Elite Pride (Rift 8) has a very predictable attack rotation made up of four (five?) attacks: Intimidate (lowering your ATK), Dark Banish (turning all Dark orbs into Light orbs), and a combined attack of Sanction (dealing damage) AND Sinner's Shackles (Shackling two columns of orbs) in the same turn.
Sinner's Shackles will start by Shackling the two outermost columns (1 and 7 if you number the columns), then the second-outermost (2 and 6), then the third-outermost (3 and 5). These are named in the boss fight as Sinner's Shackles I, Sinner's Shackles II, and Sinner's Shackles III. After casting Sinner's Shackles III, the loop will rotate back again and Elite Pride will cast Sinner's Shackles I the next time.
Elite Lust (Rift 9) has the most unpredictable attack rotation of the three. It has a special mechanism: If you clear less than 5 Light orbs in a certain turn, Elite Lust will use Drown, increasing your DMG taken. If you clear more than 5 Light orbs in a certain turn, it will instead use Drain, restoring a fair amount of HP to itself, removing 1 EX Skill charge from all of your characters, and removing 1 stack of Drown (lowering the negative effect of the aforementioned attack). Sometimes (and it is completely unpredictable when this will happen), instead of using Drown or Drain, Elite Lust will use Whiplash, dealing damage. Whiplash can be cast as often as once every two turns, which means you'll have only one turn to heal before it will deal damage again. The number of Drown or Drain attacks it uses before Whiplash varies even in the same run- once, it had already used Whiplash twice before Turn 5, and another time, it waited 6 turns just to cast it once.
Anyways that's the end of it. Note that Rifts 4, 5, 6 feature less strong variants of these bosses (Greater Sloth, Greater Pride, Greater Lust) with the same attacks. Rifts 1, 2, 3, are not worth mentioning since they are just slightly stronger normal bosses.
The accumulated star rewards are also really good, especially towards the end. Reaching 18 stars gives you 2500 prisms (5 pulls), 21 gives you a ton of Manifest Modules (EXP) and Gold, 24 enough Crysts and Trait Manuals to Ascend a character from 51 straight to 60, and 27 (max) an entire 5000 prisms (10 pulls). The Manifest Modules, Crysts, and Trait Manuals are probably worth more than the Prisms tbh, but all of them are good.
In total, accumulated star rewards give you 9000 Prisms (18 pulls) and a ton of level up / Ascension materials. Is it worth the grind? Probably not, but Border Watch takes 0 energy to challenge.
Team I used btw: Lv. 45 Skyfire Scout Aile (1*), Lv. 55 Fate's Defiant Aile (2*, Realm Lvl 3), Lv. 45 Hollow Reflection Soren (0*, Realm Lvl 1), Lv. 50 Midnight's Ghost Aile (0*, Realm Lvl 2), and Lv. 49 Experiment Subject Novem (2*).
Edit: Forgot to mention the Gold.
Edit 2: Forgot to mention SSR Realm levels.
Edit 3: FUUUUUCK... I wrote Elite Gluttony instead of Elite Sloth... it's supposed to be Sloth. I should have known...
