With normal Blackrose Prison post-U29, a fair amount of players have easily exceeded 2300 Champion Points and some even fully grinded all the way to the max 3600 Champion Points rank cap.
It is undoubtedly the best XP grind as of 2021 provided you have a partner (you should have a partner anyway to take advantage of the group XP buff).
To learn more about leveling your "alts" (alternative characters), refer to Fixed Basilisk's amazing guide that covers every aspect.
Essential Steps
With no more than one other person, both of you will need some sort of Pull ability such as Unrelenting Grip (Dragonknight) or Silver Leash (Fighters Guild). This makes it so that the adds are stacked and die much faster when compared to simply waiting for them to arrive or for you to range each one down. This is particularly useful for chaining in archers onto your designated spots as they channel.
A rule of thumb is to always stack on one side or on one elite melee add for at least the first two or so rounds of Arena 1.
You will need 150% XP scrolls or consumables with all Training gear. You should use Sigils to optimize your grind runs.
You only want to do the first four stages of the first arena. Leave the instance immediately before the 5th stage of the first arena starts so you can reset and do it again.
Round 2 vs 4 Rounds
There are some groups who prefer simply grinding Round 2 of Arena 1 repeatedly by having one person die and the other person use the Resurrection Sigil so that the dead person can immediately restart Round 2 without too much wait however this is the "lazy" method and it's been proven that doing the first 4 rounds (efficiently that is) is much better in the long run.