I just read that sometimes if you use "summon zombie dogs" at any point during the session (some people claim even having on the bar) then it will bug and disqualify you from the achievement.
I just tested this, removed Summon Zombie dogs from my bar, left game, then resumed game. I just used "Circle of Life" to summon 3 dogs, one at a time, several minutes apart, and it gave me the achievement.
So it seems they do not have to be summoned "at the same time", just exist "at the same time".
The achievement does also appear to be quite buggy.
So, in term of what spells/runes qualify for this acheivement, sounds like:
Circle of Life,
Mass Confusion with Devolution rune,
Big Bad Voodoo with Boogie Man rune
And bugs which disqualify you:
There appears to be a bug with summoning them with the Summon spell at any point in the play session?
Possibly with having the spell on your bar?
Leaving and re-entering game fixes these, or at least did for me.