I consider "Mid-Level Summons" to be of the troop type not Kings, Arch-Devils, Ice Devils, etc that are commanders/SC's. Some of my more common Favorites.

All the Seasonal Spirits have their place and position (I don't know what our cutoff for Mid-Level is here).

Summer Lions - Expensive but powerful. A good unit be bait; force attention on them while you tear at their army from a different direction.

Fall Bears - See Summer Lions, but not as effective because they do not have radiation.

Spring Hawks - The equivilant of a battlefield assassin. They are fragile but get the job done, don't expect them to survive long. Just have them do their job.

Winter Wolves - Cheap, radiation effects and ethereal. What's not to love? Of course you have to have a matching dominion or they might as well be Spring Hawks.

Spirit Mastery - 4 Death Gems for some fodder that have the chance to paralyze everything they can hit.

Lamias - Cheap cavalry type of unit that has multiple units per cast (my favorite) so it doesn't eat up too many mages time.

Mandragoras - See Lamias.

Clockwork(s) - Fodder against magic heavy nations and armies and good for defending a castle entrance. Plus they are cute.

Behemoth - No Morale Elephants for everyone.

Reanimated Archers - Fast firing free Xbows, even some disease thrown in for good luck.

Blood - I like Devils and Demon Knights personally; though if you don't have access to heavy penetrating missile using troops and do have access to Frosties or Storm Demons, they can fill a gap.