Alright, let's get this out of the way, this game isn't amazing. Definitely the weakest FromSoftware game I've played but there are some things I like. For one, it's the mechanics of 'Armored Core' and gen 3.5 too, so obviously the game is fun. However, this is basically an Arena or AC on AC focused title, and to it's credit it is very good at this so long as you are loading a 'Nexus' save.
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EDIT: THIS IS WRONG, apparently you'll be locked out of getting certain parts for doing this so I may have really messed up, lol.
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This is by far the most 'expansion' one of these 'expansion games' has ever been as it's basically a great Arena mode with tons of fun opponents and matchmaking preferences that is to be added onto 'Nexus', due to having no shop and leaving new saves to slowly gain new parts by progressing in it and the totally vapid tutorials 'Armored Core: Nine Breaker' which eventually roll the credits on the game. This is really what brings in down, in the sense that this just feels like 'Armored Core: Nexus' disc 3 and is weird as a standalone release. Because doing tutorials for hours to actually beat it? It's as suffocatingly boring as it sounds, but the Arena itself is good fun with the new balancing if you have a nice AC to load with you.

So yeah, I can't recommend it as essential playing, but it wasn't the worst thing I've played.