They're level 1. If you throw a 5th-level gestalt spellcaster in there, you WILL tpk. It doesn't matter how badly you might run him, you WILL tpk unless you're really forcing yourself not to.
Dread Necromancer 5 up one side.
Cleric 2 / Focused Conjurer 3 up the other.
At level 15:
Human Spellstitched Lich
Dread Necromancer 8 / Lich (+4LA) / whatever you want 3
Cleric 2 / Focused Conjurer 13 up the other.
Have him take Leadership at level 6 and Undead Leadership at level 9.
His cohort from Leadership should be a Human Spellstitched Necropolitan Focused Abjurer 3 / Master Specialist Abjurer 3 / Abjurant Champion 5 / Spellsword 1 / Eldritch Knight 1. If you want to Gestalt him, drop the Abjurant Champion, Spellsword, and Eldritch Knight levels, go Focused Abjurer 9 / Initiate of the Sevenfold Veil 4 // Fighter 2 / Paladin of Slaughter 2 / Rogue 3 / whatever you like 5.
His cohort from Undead Leadership should be a Human Spellstitched Death Knight Paladin of Slaughter 2 / Sorcerer 4 / Spellsword 1 / Abjurant Champion 5 / Mindbender 1 // Cleric 8 / Death Knight (+5).
You'll wipe the party every time they encounter him. Hell, with the number of minions he will have, you could very well wipe the part several times every time they encounter him. Also, each one of these has enough combat ability and enough casting to wipe the party individually, no matter what level you introduce them at.
To be honest this trio would probably just sack the city and take over.