Need to machine through case hardened 8620

Need to modify some existing parts from case hardened 8620. I am not yet sure of the case depth but estimate no more than .010". I am sure the interior is very machinable but the case was tested at HRC62 so it will eat endmills easily.

I am curious if there are any economical solutions to get through the case rather than chewing up a lot of endmills?