Dedicated summoner build isn't terribly fun, since you can only have 1 summoned monster at a time (an undead, a ghost or a demon), and your creature.
That's not to say it doesn't work... your creature and your demon can absolutely rape everything in sight late in the game (my demon killed one of the flying fortress generals in under ten seconds). So, technically you need a lot of mana, Summon Demon and a good creature... and Hide in Shadows. Pop the summons, pop the invis and watch the action from the sidelines.
But unless you pitch in with your own attacks, it is slow and boring. And when your summon expires and your creature gets killed, you have a lot of downtime.