Just a note from my experience:
Diabolos gradually builds a Stun resistance the more it is used. As RDM/DRK(w/ 1 Dark Magic Merit, 1 Thunder Acc merit, +5 Magic Acc, Dark Torque, and Abyssal Earring at the time) when I went for the CS Stun, after about 5 of them he started resisting it altogether. The RDM can use CS to have no recast timer, but Stun spam will build the resistance quickly. Luckily I realized it was resisting and was able to finish him off with some chain-nukes
Diabolos Prime was an absolute bish for me, took 9 tries to beat.