In the vein of being eaten by a tarrasque, have the rakshasa's dead body crushed under the foot of a humble ant. ;=)
However, in the spirit of the blessed crossbow bolt, maybe a weapon blessed by clerics of five (or more) good religions, or dipped in holy water from five different good religions can do the trick permanently after it is initially slain (maybe by high noon of the next day, or midnight or whatever time you feel is right)