Finished the questline in Gems of War for the ruined kingdom "Sin of Maraj".
The first thing that made it unique is that your NPC partner is Luther, the same partner (and tutorial giver) from the first kingdom in the game. He explains a bit about the Maraj's backstory while you fight off primarily Demon-typed teams, with minibosses led by demons of the Seven Deadly Sins. And my team still largely has this weird pattern of having less difficulty with the actual "boss" battles than the "lesser" battles preceding them.
I really should start looking into Freeze sometime, to reduce the CPU's ability to farm extra turns.
Anyway, what is the final boss of the area? None other than Plague, War, Famine, and Death.
Yup. And hell, as a boss battle The Four Horsemen of the Apocalypse (their home kingdom as stated on their cards is literally the "Apocalypse") do not disappoint.
1 - Their base rarity is Mythic, and at Lv.25 they are over cap.
2 - Their spells require over 20 units of Mana, but each one can collect three(!) colors of Mana.
3 - All of them have some a nefarious final passive buff: Plague debuffs your entire team by 1 point on matches of 4 or more (and is itself immune to status ailments), War's Attack stat increases 3 points per turn (and its spell not only Burns your whole team but scales with its own Attack, ouch), Famine debuffs your party leader by 2 points per turn (plus regens 1 Armor per turn, and its spell drains all mana from your team to strike one target for massive damage), and Death drains 2 Life from your leader per turn (not to mention its spell deals damage scaled with its Life, ouch).
4 - Fortunately, I did have at least some counters already on team: I've classed my hero as a Stormcaller with +2HP regen per turn (conveniently heals off the damage from Death's ability, though its HP still increases over time), plus the Warlord class weapon's ability to scale damage by the target's Attack enables me to literally one-shot War about five turns in. (War isn't even first in the party formation, so its increasing Attack stat isn't even a threat until its spell is charged.) Also, everyone except Famine is at least partly Blue Mana, so Amira (who is finally leveled up) can two-shot any of them with her skill, so long as I can collect Mana quickly enough. ...and that was the main problem. I don't have any team members ready who can generate Purple gems on the board (though I have seen quite a few by now), and as usual, I can't afford to let the CPU get any extra turns.
I didn't bother counting the number of times I had to retreat from the battle because I could tell it was going badly and fast (or persisted through to actual defeat). I did eventually beat them, but it was definitely a lucky victory.
Yes, I know; I could have trained up new team members with different skills or buffs (double damage vs. Demons, for example) but I play collectible games with a "Pokemon rule" -- if you can't proceed with team members that YOU personally like then what's even the point? With so many options you should be able to make almost anything viable.