Summoners can be found in Campaigns 2 and 3, in the following levels. Note: Playing "Crucible of Evil" is recommended, as you can find at least eight Summoners there that are easier to kill than in "The Keep".

At the end of "Gateway to Hell".
"Crucible of Evil".
"Terminal".
"Tower of Hellfire".
Lying in the hospital and not summoning zombies in "City of Ashes".
"Forest of Corpses" "The Keep" "Army of Darkness".