Unlike vanilla, there are only 4 R-Series in the Factory, not 6.

R-Series will begin counterattacking when there are only 2 left. The more R-series that go down, the stronger the counter attacks become. When there is only one party member left, they will counter with laser attacks. When only one R-Series remains, it will counter with party damage. High-damaging magic spells like Flare or Dark Matter that are likely to defeat all of them in one shot is the preferred strategy.