Armored Core For Answer has the most time consuming online achievements IMO, the main ones break down like this:
Online Kills: 1000 / 2000 / 3000 / 4000 / 5000 / 7500 / 10,000
Online Wins: 1000 / 2000 / 3000 / 4000 / 5000 / 7500 / 10,000
Also the online population is full of good players so I would say getting 10,000 kills/wins is very difficult.
I gave up on 480/1000 in the end.