I'm feeling a bit creative today, let's see here...
Approach:
When mc attempts to gain control over someone's mind, a battle with that person will be initiated inside their head.
A mental-world battlefield, so to speak.
It can be its own unique game, and have its own unique gameplay and rules and differ from person to person.
*The person is not aware that someone is trying to take over them, but will automatically attempt to boot the invader from their mind with every ounce of mental fortitude they can muster.
*Mentally stronger targets offer greater challenge.
*The mc is temporarily transported into the head of the person they wish to establish their control over.
*Mc's objective is to make it to the central control node and take it over, thus establishing permanent control.
Central node is to offer the biggest challenge.
*Other minor nodes exist, each offers its own unique challenge, each acts as a door that needs to be unlocked to get to the central control node.
*Unique very strong mentally individuals can exist who can even attempt to retaliate by invading mc's mind through their mental link, can make mc's invasion harder or even impossible for the mc to accomplish without some way to weaken that person's mental defenses first. (Making them drunk in the real world, perhaps? Mc becoming more capable at using their powers? Etc.)
*The invasion doesn't have to succeed in one go, mc can gradually weaken the person and slowly take over node after node over a long period of time, this will also be reflected in that person's behavior in real world. (They will become more agreeable than before to certain proposals and such, for example.)
*Mc can discover various secrets while invading someone's mind, he can then use those secrets in various ways against that person in the real world. (To weaken their mental defenses further in some way, to blackmail them, etc.)
Hm~ This will do to start you, use your own head for the rest.
EDIT: And damn, missed the bleedin' date. Freakin' necromancers.
Oh well, whoever discovers this momentary inspiration of mine can have it.