Keep in mind, including Miles makes him a target to the IBM and Kei a target to the Walrider.
Kei can paralyze Miles and make him easy work for it, and Kei is known to have an extraordinary amount of black matter, he is able to summon it more than once a day. (We'll limit it to 2 times for the sake of things, as 9 would be kinda silly.)
I think killing the Ajin can lead to the summoning of numerous IBM ghosts at a time, dependent on how the Ajin is killed. Kei is can regen has at least Mid regen, but when subjected to full fledged decapitation, his original personality dies in the process.
I think Kei and the IBM can easily take out Miles well under an hour, but it becomes a matter of whether if the Walrider we'll eventually die or if it can use Kei as a host.