Dark Souls again. This time it's the Four Kings; magical once-human monstrosities found in the flooded ruins of New Londo.
Basically, you have to be super aggressive and get each King's health down fast because of how quickly they spawn. It starts out with one popping up, but every thirty seconds or so, another one spawns, and this keeps on until their collective health is whittled down. Each King isn't a particularly tough opponent, but trying to keep track of all four at once is nigh-impossible, and their AI is pretty relentless. Also, the NPC summon for that fight only appears if you summoned her for a previous boss fight, and even if you can summon her, she tends to get stuck at the top of the staircase into the boss room firing magic at an enemy she's incapable of damaging. Even if you can get her into the boss room, she's mostly only good as a meat shield.
Also, they put the fight in the Abyss, which is pitch black and devoid of all landmarks unless someone left a message there. Seriously, it's impossible to gauge how far a King is from you until you bump into the fucker, or more commonly, his sword bumps you. Speaking of the swords, all their attacks are purely magic-based, which isn't immediately obvious and makes it difficult to defend against unless you're kitted out with the right gear.