(2/2) You can boost off the arms and do tricks, you can boost off the spikes at the top and do tricks, you can boost about anywhere on Asura and do tricks.
My personal method is boosting off of one of the spikes, boosting back on top while in the air and then boosting off of one of the spikes again when the arm is raised, as this allows for Asura to slam it down while doing tricks in the air, allowing for me to climb back up and repeat the trick without much wait.

(1/2) Like seriously, there are so many places where you can boost off of Asura with the ability to do air tricks, it isn't funny.
So far, I'd even say that Asura is among the most consistent, limitless and easily accessible way of grinding skill pieces.
Like, sure, you've got your breakables with about a thousand skill points in them, but those can be hard to come by and take a while to show back up, and you've got your glitches where if you hit just the right spot in a wall, you can get stuck while still able to do air tricks, effectively creating an infinite skill piece grinder, but Asura is easy, consistent and even if you do go about killing both of them, a random event with the shooting stars or whatever will revive them.