Instructions
Open a Town Portal just outside of the battle area. If Diablo comes within range of your portal, he will cast Bone Cage around it, making the Town Portal inaccessible to your Druid.
When Diablo first appears, fall back. Attempt to lure his minions away and isolate them. Confronting both Diablo and his minions at the same time is suicide. As his minions are made up of the same creatures you've been killing the entire act, use the strategies that have gotten you this far.
Send your minions against Diablo once his own minions have been defeated. Use whatever you have -- wolves, bears, raven, your mercenary, it doesn't matter. As they attack, cast your most powerful elemental attacks. If you've chosen not to put skill points into elemental magic, use a ranged weapon to attack Diablo from behind your minions.
Return to town using the Town Portal when your minions die. Resurrect your mercenary by speaking to Tyreal, resummon your minions, and heal and restore your mana.
Use the Town Portal to return to the battle ground.
Open a new Town Portal outside of Diablo's range.
Repeat these steps until Diablo has died. As a Druid, this is a time-consuming battle of attrition. Diablo will slaughter your minions, you will return to town, resupply, come back and peck away at him again. Eventually he will go down.