This is stupid and all but useless but it is just a little glitch.
If you get diablo to phase 2 (shadow realm), and then get him to like 10% health and wait for him to bone prison. Let the bone prison catch you and then have your party or if you're a wiz, let your venom hydra kill him before he grabs you up. The phase will change to the last phase of the fight and you will be perma stunned till the fights ends.
yeah I know it's stupid but meh.