Simulacrum should aim where you're aiming, hitting the shadow clone in the process. With simulacrum active run away from the shadow clone, it will probably still aggro you but this is fine, just run until you, the shadow clone, and your simulacrum are in a nice line (see video), then spam bone spear while aiming away from the shadow clone. Damage Diablo enough that you go to the shadow realm, then once the shadow clone spawns, use simulacrum. I used Blighted Marrow, but several other runes will be fine. Next, make sure you have simulacrum and bone spear equipped before you fight Diablo, and that the rune for bone spear doesn't modify the behavior too much (it should still be a spear of bone flying in a roughly straight line). For me, I found hard to be enough to keep things alive long enough that there weren't accidents but keep things with low enough health that you don't need to try. With all that out of the way my favored strategy for this is as follows (see below video for a demo): First off, reduce the difficulty far enough down that things die quickly, but not instantly (more than one hit) from your basic resource generating attack.

3) Unequip any legendary gems that either directly deal damage or apply DOTs. You can see your thorns damage under 'character details' from the inventory menu. 2) Unequip all thorns giving skills, gear, gems, etc.

The first thing is some things to try if you find yourself having trouble getting this to pop: 1) If you have Frailty equipped, don't use it on the Shadow Clone. Since I was noticing in comments people were having trouble with this still, here's how I got the achievement.

Diablo 3 necromancer achievements