Akachi is a Level 30 Cleric/5 Fighter, but he doesn't cast spells - just a small selection of spell-like abilities (the worst is Isaac's Missile Storm, which comes out rarely). Honestly it's his six attacks a round with a wicked-looking negative-energy-dealing scythe that's the bigger worry. He's also immune to most status effects and is permanently Hasted. The fight is more or less a straight-up beatdown. There's no gimmicks: no levers, no statues, no rituals, no portals. However, Akachi has 700 HP... ...and every time he reaches Near Death, he heals up and spawns a little Akachi to help him. These Akachi clones are a lot weaker than the real thing, with 'only' 360 HP and all 18s in their stats instead of all 30s, but they're still a menace. And if you're not careful, you can end up surrounded by the things - a very painful experience.
There are four Akachi clones to fight through before the Faceless Man can be defeated: Remorseless Instinct (chance to Confuse on hit), Infinite Despair (Doom on hit), Unrelenting Fury (Hasted) and Insatiable Hunger (Vampiric Regeneration +5). After they're dead, it's only a matter of finishing off Akachi.