Of course the Moonlight Greatsword is in Armored Core 6

Armored Core 6: Fires of Rubicon continues FromSoftware's long tradition of finding a way to include the Moonlight Greatsword in its games. Despite this not being a fantasy game where swords naturally belong, Fires of Rubicon does indeed have the famed Moonlight Greatsword.

Or, as it's referred to in-game: Light Wave Blade: IA-C01W2. The Moonlight Greatsword has been in every game since the original King's Field, though its most famous appearance is in Bloodborne (where it was used by a particular boss). It's a giant sword with a distinct design and a blue glow. It typically also fires off blue energy waves with every attack.

In Armored Core 6, the Moonlight Greatsword can only be found inside a chest, which sits in a lake towards the end of the final mission of chapter 4 (the one with the boss fight). The chest is guarded by four enemies, but you could always grab it and boost away without fighting them.

The video below from Arekkz Gaming shows the whole run, and what its attacks look like in action. And yes, it also sends off waves of blue energy, making it play two roles (ranged-melee).

If you don't feel the need to rely on such crude weaponry, perhaps you'd prefer to... punch your enemies instead. For more sensible advice (especially if you're new to the series), our Armored Core 6 beginners tips is a read you won't regret.