[Nikelaos]

Thaumaturgy 9, D7, Undead mastery
Alteration 9, S7, Arcane domination
Thaumaturgy 9, S8, Master enslave

okay not worth going to the above extremes for this but these are the obvious ways to charm both the ancient presence and the mandragoras though you should consider scripting retreat incase the ancient precense resists your spell. If in the likely case you aren't in a position to use these spells then get a bunch of mages with a few units to act as a meat sheild spamming any of the following lower lvl spells.

Thaumaturgy 6, S4, Enslave mind
Alteration 6, S3, Control
Blood 5, B2, Hellbind heart
Thaumaturgy 5, D2, Control the dead

Obviously if you aren't interested in adding some mandragoras to your garrison then by all means kill some off with preists spamming banishment(becareful not to hit ancient presence) before charming your target with the spells listed above.