It can be done incrementally.
If you can alter people's memories at your whim, then you essentially have control over their sanity.
You say that the practitioner can alter, add, or delete memories, so I will give an example for each. You'll see a pattern.
Alter memories of decreasing triviality. Start with memories that aren't likely to be accessed regularly, such as the name of a childhood acquaintance. It'd be best if there were multiple memories involving the changed object, as this will cause an internal discord. Slowly, the target will start to feel as if they're getting forgetful, or that their mind is overworked. As this mental fatigue grows, the memories that are altered can become increasingly more important to the target. The goal is to drive the person to insanity by making them doubt their own mind and eventually their perception of reality.
Remove memories, first randomly, then focus on specifics. The first memories removed can be randomly selected, but they also shouldn't have a major impact on the target's life. They'll forget things like who they ate with yesterday, people's names, maybe appointments. As time goes on more memories can be removed during each interaction. The target will start to believe they're having general memory issues, with no reason to associate the issues with the practitioner. Once they sufficiently doubt their memory, you can remove specific areas of memory (such as their professional skills, or a family member).
Add memories that increasingly conflict with reality. This will run similar to the altering their memories tact. The difference is that they'll have multiple memories of the same event, or remember different events that would be occurring at the same time.
Alternatively, you can build up a severe paranoia/phobia, by slowly adding memories of being traumatized by something. A childhood fear of the dark could be expanded upon. After every visit they have a new memory of something frightening or terrible happening during in the dark. As their mind processes these memories, their phobia grows until they start avoiding the dark. This phobia, using the mind's own power against itself, could naturally lead to hallucinations and severe anxiety. (Hearing sounds in dark and being terrified. Thinking you see shadows in the corners of your eyes.)
Update: Since the practitioner will only have a single opportunity to affect the target, I would suggest a few alternatives to those above.
Completely alter all memories of specific people to be about other people. This is Goodkind-inspired, a sort of reverse Chainfire "spell". Everyone else will remember events how they actually happened, but the target will remember them as if they happened with somebody else.
A good friend could be completely replaced by a number of other individuals. As more events are discussed with others, the more the discontinuity will show. Once the target runs into a person they've "forgotten", the cracks in the psyche will expand. The key is to not just remove the memories of the people. The target should be able to argue "No, Bob wasn't there, it was Tom."
One completely imaginary person should be woven throughout all of these altered memories. Like a fake childhood friend that has always been in the target's life. Once the target starts insisting, "Just ask Charlie, he was there, he can explain all of this," then people will really start to think he's crazy. Eventually, they'll convince him of it.
Add/remove/change a large number of memories to create acute phobias. Change even childhood memories. If there was an incident where they were afraid, but a parent or friend soothed their fear, remove the memory of that soothing and replace it with continued fear. Phobias are irrational fears, so the planted memories will need to make that fear seem rational. They could remember the thing frightening them, or remember other people talking about how the thing "got" someone else. Injuries they've have can be remember as being caused by the thing.
I'd use a "timeline" like this:
Child-like fears reinforced during young childhood memories.
Older child memories have the child-like fears go away, but that lingering doubt is reinforced.
As the person "ages", the memories increasingly convince him that the fear is rational.
In adulthood, the most recent traumatic memories are altered to be caused by the fear.
In recent memory, events that couldn't have really happened are added (somewhat like hallucinations) to make it feel as if the fear is growing in power.
The severe phobia(s) will present themselves readily. Anytime something negative happens, their mind will process it subconsciously with these new memories. Their fundamental worldview has changed, and they won't have any way to tell that those fears are unfounded. Severe phobias can cripple people, or leave them confined to their home.